Maybe sponsons. Here is a thread from paddle24seven that talks a bit about it.
http://paddle24seven.com/index.php?option=com_fireboard&Itemid=82&func=view&id=1... They will add some secondary stability without mortgaging speed. These will not make the canoe as stable as outriggers, but they will add a lot of stability.
Hut,
Wally
Los Humungos Paddleos
Oh, and do not add balast. If you go that route, just lower the seat.