In reply to Reader_Rambles:
Crusher Holds make a plate that clamps a doorframe - that or something like it could work in a lot of locations and is dead simple to make from some ply.
Hinges in the middle of a length of 2x4 will always be a liability wether inline or on the side. You may be better off thinking along the lines of easily dismantled rather than collapsible, using big coach bolts and nuts to clamp overlapping sections of wood together.
When I built my outdoor wall it was basically an A frame with a big coach bolt for each pivot, and the horizontal was a rope that ran from the front leg to the back leg through holes in the wood.
I reckon if you made something similar, but with sectional legs it'd do the job. To get a height of around 1.8m off the floor at the board you'd be looking at legs around 2.4m long.
To make the legs, chop a 3m 2x4 (or 2x6 if weight matters less) into ~1m lengths (not quite your 2ft, but more shorter sections will probably still work) and either do a half lap joint with a pair of big bolts and washers, or go simple and overlap the ends about 20cm wide edge to wide edge and bolt them together, again with a pair of bolts to keep them straight.
Get some sections of rope and use them to keep the legs the right distance apart and maybe stop them splaying out as well. The top sections of 2 of the legs will need some additional wood spanning between them (again bolted in place), and triangles built out to attach a bit of ply vertically for the hang board.
Broken down you'd be left with 12 1m lengths of wood, a triangular boxy thing with the hang board, and some nuts and bolts that you could store in the top behind the hangboard along with a pair of spanners.
A similar design could be adapted to make a design like you're describing, with a smaller base triangle and vertical uprights, I think it'd be slightly less material.
Left field approach, buy a pile of scaffold poles (lightweight aluminium ones are available), some inline and angled connectors, and big saddle clips to fasten the ply and you've got a lightweight metal version.
Post edited at 19:05