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
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.