is it possible to assign a custom inventory to a shopkeeper mob so they'll sell objects you've created?

Set the mob as a shopkeeper.
Give them the items you want them to sell.
Instaroom.
Save the area.

Should be good to go.

If we're talking about shops created by builders you'll also want to set the inventory flag on any items the shop sells so they don't run out of stock on their regular items. Otherwise you'll have to make sure the mob loads a ton of each object to have a decent number of items in stock.

Uhm. I don't think you need to set the item as inventory. The code applies it automatically when the item is reset into the shop. And if SWR/SWFotE FUSS aren't, well, then that needs fixed.

You are correct sir. They do indeed set it automatically.