Pickpocketing summons not dropping anything seems just fine to me. Summoned creatures are summoned only carrying the bare minimum that defines them. If you summon "a swordsman", he comes with sword and armor and everything else that makes him a fully-fledged swordsman. If you summon a ratling archer, he comes with a light crossbow and quarrels etc. The pockets are empty because swordsmen don't need anything in them to be swordsmen. Even if they had money wherever they came from, it didn't get summoned alongside them.
As far as regular item drops go, I guess I'll agree with JellySlayer that "none at all" seems like too harsh a solution, especially for those summons that pose a credible danger. Having no items at all from killing hordes of monsters is not fun, and exploitable is still preferable to not fun.
Though I have to stress that greater undead vaults don't make the best of examples, as they were never enjoyable to clear except for bragging rights or artifact collectomania in ADOM 1.1.1. Whether the hordes of summoned creatures drop items or not, the chore of slogging through them, the potential danger of deadly summons and the time spent always vastly overshadowed the reward. Just in general lich kings and emperor liches are seriously bad news.
(Please don't try to convince me that pickpocketing (summoned monsters, or in general) is fun. It's not going to work and I'm not that important anyway. You might convince others and many won't need convincing, for whatever earthly reason.)