Regarding whether it's possible to have a canoe on top, yes that's possible. I've been looking into roof-top tents and it seems three models from Autohome do this:
- AutoHome Maggiolina (Extreme, Grand Tour and Safari models) have roof bars so you mount stuff on top.
- Same goes for AutoHome Columbus.
- The AutoHome Overcamp can mount on the side so there's place on the rack for a canoe.
For all the models (and pictures with canoes!), check
www.autohome.it - for the models that are available in US, check
www.autohomeus.com. They apparently don't have all the models available in the US, but maybe ask them. (Btw, I am not in any way related to Autohome.)