Do-able...
Dear Michael,
In order of appearance:
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What I'd like to do next is to install some orange LEDs at the firebox to give that flicker effect.
Do-able. SMD 0603 LED + (SMD?) resistor, fed from decoder.
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The main issue is that I can see is that the 8 wires on the loco/tank connector are already accounted for, 2 track, 2 motor, 2 for the headlight and ground, and 2 for the boiler speaker so I can't use the Decoder Buddy in this instance for the additional functions, unless I wanted to add additional connections between the loco and tank which I'd prefer to avoid.
First, doesn't sound like a DecoderBuddy limit, but a Loco<> Tender wiring loom limit...
(I'm sure the DB has more "pins"/solder-points available... 
Second, yes, Loco<> Tender looms suck, which is why on my 3-truck shays, I do everything I can to eliminate them completely.
- Both "Loco" trucks + sound decoder + KA + speaker + lighting in the "Loco" unit,
- a TCS FLx + KA wired to the "3rd truck" wheels, feeding the backup headlight, completely standalone.
Set both decoders to the same loco address, and the end-result is "seamless" in operation...
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My thought is to use a lighting accessory decoder (TCS FL2 or FL4 or similar) in the boiler which could then provide control for the firebox flicker (and maybe even marker lights if I'm brave enough).
Do-able, see above. NB that the FL4, despite having 4x Lighting outputs, is actually smaller than the FL2!
(Space saving FTW!). That extra space however will likely be eaten up by a 2nd (smaller capacity) KA. Suggest reading up on SMD Tantalum capacitors and DIY KAs
https://model-railroad-hobbyist.com/magazine/running-extra/2018-12/stay-alive-n
Also worth noting that because the FLx is NOT the "motor decoder", it has no access to the BEMF from the motor. Thus any "firebox flicker" will NOT change dynamically in relation to speed or load...
IE If your existing "firebox flicker" examples you are happy-with have noticable "flare up" as the loco starts climbing a grade, or "flare flare flare" as the sound goes "chuff chuff chuff", then don't expect the same from a "firebox flicker" generated by a secondary/"aux lighting" decoder...
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I assume that it doesn't matter how the decoder is addressed and setting it the same as the loco proper wouldn't cause issues?
Decoders are (relatively) dumb devices. They sit on the "network", and if they get a command addressed to them, they "just do it"... so while generally having multiple LOCOs with common address is a recipe for confusion, In This Case having the "Loco decoder" and "aux lighting decoder" set to the same address is entirely appropriate...
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I don't see a lot of mentions about doing this but is it as simple as I think it to be with regards to programming?
Ah ha! Here's the rub! (No free lunches!)
Programming as part of initial installation can be relatively simple. Pre-programming a TCS FLx "standalone" for a given task is a doddle, and with it pre-configured, the installation becomes simply a matter of "bolting/soldering it in"...
HOWEVER....
Once you have Both Decoders wired into the loco (in parallel relative to the loco pickups),
programming becomes a Bear Of A Thing, as there is no way the Decoders can determine "was that change to CV nnn for You, or for Me???"
NB in the case of my Shays above, I can easily separate the "Loco" and the "3rd truck bunker", and program them completely individually, so the below is not required in My Case. However, I have tested the below on an Athearn SD40-2 where an N scale ST TSU provided Motor + HeadLights + Sound, and a TCS FL2 provided additional "ditchlights"...
There IS a "Decoder Lock" function built into NMRA spec decoders, which essentially gives each decoder an ID number (CV 16). There is a 2nd related CV (CV 15) which is effectively an "enable/disable decoder for programming" switch.
https://docs.tcsdcc.com/wiki/Support_-_Decoder_Lock
The Recipe is as follows.
(Assuming ALL "Initial Programming" of BOTH decoders is complete, but they are NOT yet "installed/wired together"!)
- Loco (Inc Sound) decoder : Set CV 16 = 2
- Aux Lighting decoder : Set CV 16 = 3
At this point, both decoders will appear to stop accepting Programming Instructions. This is Expected!
- Now, do the physical install, and wire everything up Like You Mean It, as if It's Forever...
This is NOT the time to scrimp, take shortcuts, or otherwise do a 1/2-hearted job!!!
You do NOT want to have to re-open this nightmare of a shoehorn-wiring job again!!!
- Now, set CV 15 in BOTH decoders to "7" (Effectively Locks both decoders)
NB that with BOTH decoders wired together, and BOTH decoders having the same Loco Address,
programming CV 15 in "Program on Main" mode One-Time will effectively program BOTH decoders CV15 value simultaneously....
This is the One and Only time when "programming a given CV on 2 decoders simultaneously" will be a Good and Expected Thing!
From Here, assuming all is well, there should be no further need to tweak anything,
the loco just operates exactly the way you wish, and all is sunshine and rainbows and lollypops and trains...
...until you DO need to reconfigure something, at which point:
(Assuming "Programming on the Main" mode)
- Program CV15 = 2 (The Loco decoder will UNlock for programming, as CV15 = CV16)
OR
- Program CV15 = 3 (The Aux Lighting decoder will UNlock for programming, as CV15 = CV16)
...then tweak CVs on the respective decoder as required,
and don't-forget to set CV15 back to "7" once you're done...
If the above sounds like too-much-angst, then wiring the TCS FLx with a 2pin plug so it can be Unplugged/Isolated from "the loco" and programmed seperately may be another option...
...albeit accessing that plug, let alone getting at the decoder in any meaningful way, is exactly what you're trying to avoid in the first place...
I hope this helps...
Happy Modelling,
Aim to Improve,
Prof Klyzlr
