like you thoughts mate was thinking of the same a while ago
If you can have a carpet truck mount , then why not a gutter one
Its going to have to be a hell of a vac though
if you look at the carpet side of things the suction is made at the point of contact with the carpet , and that is only a small amount of water being sucked up
This might change with the amount of muck in the gutter and the distance it has to travel back to the vac
have a look at this it might be what you would need
Andy
http://www.bigbrute.co.uk/demos/bulk-bin/index.htm