My take, this comes down to a question of Cover vs Concealment which T20 doesn't seem to get into except an oblique reference in the vehicle design section. Fortunately T20 is d20 and the topic is covered in the D&D 3rd Edition.
Basically Cover is hard, Concealment is not. In my opinion a body in armor is Cover, one not in armor is Concealment.
I'd say Cover adds to AC, about 1 point per 10% being a good average. Being that T20 uses AR as well you do need some idea of the AR of any cover like you say. Check the Errata for the AR Table ed.1 pg.158 for numbers. As for an unarmored body even if you called it Cover, I don't see it adding to AR since your PC's and NPC's don't have a natural AR.
Concealment is more like a saving throw. If your attack would normally hit the target but they have some Concealment there is a chance that you actually missed (or in this case hit the body slung over his back). About a 10% chance to miss per 25% hidden for a good rule of thumb.
That's how I'd handle it, and of course the NPC should have been penalized for the added encumbrance of the dead body, hope your GM at least figured that into it but I have to wonder from the description of the NPC running away
Lumbering maybe, unless they were very strong and not already encumbered. There are other combat modifiers that may have come into play that are not mentioned (due to the license) in the T20 book that you will find in a Core d20 book. Stuff like the max Dex mods for being encumbered.
Or you could treat it like firing into melee (Mr. Body is grappled by the guy carrying him) only in this case you don't care if you hit friendlies (i.e. Mr. Body) so there's no negs on the attack, just a percentage breakdown of where the succesful attacks hit, 25% it hits Mr. Body, 75% it hits the guy carrying him. For an even better bonus (though not in the spirit of the system and not endorsed by me) aim at the dead guy (all the munchkins will), aw come on, he won't care, he's already dead