while I am not doing any modding myself, I enjoy looking through the files to see how things work. that being said..

story/rawfiles/goals/trade.txt

has this bit

Code
PROC
GenTradeItems((CHARACTER)_Player,(CHARACTER)_Trader)
AND
NOT DB_TraderGeneratedTreasureForLevel(_Trader,_)
THEN
DB_TraderGeneratedTreasureForLevel(_Trader,0);


it seems more than reasonable to assume you can rig something up to restock merchants. however as I said, I haven't done any modding so it may be harder than it seems.