I have no idea how these gundam things go together, I have no skill in model design, least of all poseable models.... but I thought i'd throw a few ideas into the "poseable joints" discussion.
I should also add I have no idea what your joints look like or if my ideas would apply to them...
Basically, both my ideas work together, one or both might do the trick, or neither will work at all...
Also they involve using rubber, not paper.
What if you were to add rubber "gaskets" or something to each half of the moving joint, so that it could move, but the friction/grip from the rubber would stop the model from collapsing on it's own weight. I'm talking something like the rubber they make household and surgical gloves out of, with the "slippery" powder removed of course.
If that didn't hold itself together well enough to grip, how about using rubber bands inside the parts themselves, or the ballpoint pen spring as Lex mentioned earlier?
Not sure if I explained it properly or not, but if you're interested in the idea I could try to draw something up to explain it a bit better.
Just thought I'd share that before it gets buried in that dark corner of my mind where all the things the Mrs wants me to do go to be forgotten about.