I found a similar bit of code in Vic's script, the node number is the same...
Node 1009 must be the one used to have a party member put his armour away, via the p.m. interface... There must be something similar to have them WEAR armour... Always operated via the same interface: Marcus probably lacks this bit.
I know critters pick up and use weapons they can use: for example Dude_Object saw a gecko pick up and equip brass knuckles. I don't know if critters can do the same with armours.
Apart from scripting there's no apparent reason why a proto should not put an armour on. You can have them wear one giving it manually, at least. There's no such thing as size defined in the headers (unlke in Arcanum, for example)... one size fits all, so to speack. I'm positive this is script-related.
See you.