The answer is no in both cases. Let me check with stock outfit, just in case...
Edit:
Doesn't work either with all the stock items I tried! 
Edit 2:
- helmets: OK
- armors: OK
- boots: OK
- belts: never OK in game, always OK in the toolset.
The difference between these four types of objects is basically that the first three use the "armor" template (with the armor parts available in the properties: chest, gloves, helm, boots, belt) whereas the belt object uses the typical object template (with the three object parts selection).
I therefore tried with the Part 1 number xx referencing the mdb number xx - 1 (P_HH?_CL_BELTxx), but still no success. Using the armor template and selecting "belt" is not enough (if I keep "armor" as the item base, it goes to the armor slot; if I then change it to a "Belt" item, it remains invisible in game).
As far as I can tell, "belt" was supposed to behave like helm, glove, boot, but in the end is only another object like amulet, ring (i.e. gives the properties it's supposed to when worn or carried, but is not displayed).
Tchos, do you remember which outfits worked? Before I switch to the armor / armor with belt swapping with the belt equip/unequip events, maybe these could show what I'm doing wrong (though I reused stock items...)