If the food disappears from their stomachs, it won't help too much. More likely he'd have to summon the tools to make a farm and such. I think any attempt to word it so that it nixes all potential abuses is probably impossible, and you're better off with something along the lines of "what is appropriate or not for the task is up to the DM" or something.

As for Silentium Adjustments, I'd just keep them minor and try to over come some of the weaknesses of the Ring slots. Stuff like ability enhancers, resistances, self-repair (Belt of Healing) might work reasonably well. I'd probably avoid letting them get enchanted, but then maybe that's a good idea - certainly saves you a lot of trouble coming up with stuff yourself.