Sram X5 RD limit screw A RD was recently lightly smashed in. I straightened the derailleur hanger "by hand" and now it is "good enough" with some chain rub on the cogs. I set out to readjust the rear gears but am facing the following problem: no matter what I do, the chain is never prevented from dropping between the cogs and spokes at low gear ratio.

I took a closer look and while the high limit screw rests against a wide platform on the body of the RD, the low limit screw seems to resist against the thin wall of the parallelogram!

My hypothesis is that the parallelogram is bent and "too open", so a suitable repair would be to bend it inward a little so that the screw makes contact. But that sounds too sketchy - why wouldn't there be a platform for the screw to resist against? Am I in error?
You're quite correct, the limit screws rest against the thin metal of the parallelogram arms, and these have been bent in your case. Good catch.

It is the same design on my SRAM X7 rear mech, and presumably in many other designs as manufacturers balance cost, weight, and perfomance in normal use. 

My Shimano RD5700 has the screw resting against a slightly stronger looking position, closer the the root end of the plates they rest against where the plates would be stiffer. This looks like they would be more resilient to the kind of bending damage you have suffered, however I once replaced a Tiagra RD where the screw had punched through the metal of the mech and become inoperative/irrepairable. So that failed in a crash too.

You could say someone used the mech in a situation 'beyond its design capabilities'. If you can bend it back so the plates are straighter, the limit screws should work again, and the geometry of the mech may even be a little better. The next question might be whether the bending damage and re-alignment will significantly weaken the part, leading to failure and more expensive repair balanced against cost of replacement now.