Yeah, feebleminded tart commanders could be equipped to get respectable MR. It's the ones that you didn't GoR that would be in a more difficult situation. For instance, without magic it's almost certainly not worth GoRing a monstrum - but they are useful troops. A feebleminded monstrum would be at 13 MR, that you could only boost as high as 17 MR with antimagic/iron will/army of gold (excluding magic scales in this of course). At the same time its high HP would make it a target.
But perhaps a way around this would be to give the monstrum in particular a starting MR higher than 18? If it weren't feebleminded and were GoRed this still wouldn't be a problem since it has such limited slots compared to the other tarts. In fact you might have to give it a perk like exceptionally high MR to convince anyone to GoR a monstrum without magic.
Interesting idea about the siege golem, I'd never considered GoRing one. Those attack/defense stats look pretty horrible (just the kind of target I like to send my glamoured thugs after
) but I guess it's not much worse than a golem and if they had hand slots you could get that looking a little better.