What about pocket legs?

I use friction fit pocket legs on my layout.

Banquet table legs

Lee Valley folding leg

Lee Valley folding leg brackets

< http://www.leevalley.com/en/hardware/page.aspx?p=40035&cat=3,43586>

Or another version is by Platt River

< http://woodworker.com/heavy-duty-folding-wooden-leg-fitting-mssu-115-637.asp>

NTrak Site

Hi Bill,

Go to the NTrak website, I believe they still have folding leg instructions.

PVC pipe?

I started building a few module frames the other day, but haven't gotten to the legs yet and was considering using either 1" o 1-1/4" PVC pipe. It's inexpensive, lightweight, consistent in size and not prone to warping or swelling/shrinking like wood.

Careful consideration.

Or another version is by Platt River

http://woodworker.com/heavy-duty-folding-wooden-leg-fitting-mssu-115-637.asp

These do work nice, but the mounting location needs to be rock solid. Those who use them in our group have mounted them to the supporting shelf along the faceplates and unfortunately those mounting locations with repeated use become somewhat loose. Just an FYI.

OT; I personally like the idea of the pockets for individual legs and use Velcro to secure the legs to the underside of the module when not in use. 

Our club used tables for the O scale modules

They came from Sam's Club and break down pretty small.

http://www.samsclub.com/sams/4-light-adj-light-comm-adj/prod8340207.ip

We added a small (1 x 1) strip on the bottom of the modules to keep them from sliding off the tables. The tables come up to a reasonable height (counter - about 39 inches or 1 meter).

extruded aluminum profiles

You might want to take a look at extruded aluminum profiles or T-Slot profiles.
I have started to use this stuff to make the bases for my T-Trak modules I take to my local shows.
The modules sit on the banquet tables so the legs are very short but this is a very versatile product and with a little bit of imagination it wouldn’t be too hard to design folding legs. For my modules I use a metric size 20mm x 20mm.  for your legs I might suggest 20mm x 40mm.  Manufacturers offer various connectors and hinges that should help as well.
Depending on where you live you could try:

Misumi usa:  http://us.misumi-ec.com/vona2/mech/   Look for Aluminum Extrusions near the bottom of the list on the left side.

80/20: http://www.8020.net/  A lot of people sell their stuff on eBay. There are a lot of other suppliers that have different products and other accessories. Hope these pictures come out ok.                                                                                                                         

3-in PVC

I'm using 3-in PVC pipe with flanges on my hollow core door peninsula(s).  Not foldable but removable as long as you don't glue the things.  LOL.

This is a semi-permanent layout though, so I have no experience with repeatedly removing and reseating the pipes in the flanges.  I suppose they could wear to the point of getting wobbly in time.