What should I use to fix chips in veneer plaster?
I'd use veneer plaster. Your instinct about joint compound and spackling is on target, and you might get Plaster of Paris to work if you get the mix perfect - but why bother.  

My guess is that the people you've been talking to at hardware stores either don't have a clue what you are looking for or want to direct you to something in stock.  I'd check at a building supply store (you might get lucky at a big box store, especially if they carry blue board) that contractors in your area use.

Veneer plaster is extremely uncommon in my area, and I can get it when I need it from any of the big chain stores (although I usually have to order it - they never stock it).