Paul
as far as I am aware we have no "river rules" for open canoes.
Applying the river rules you need 3, 3*'s - which should probably be open canoe 3*'s so you need to pusuade the 3*'s to go out but otherwise I see no reason why not. I know that when Anton worked at the basin he used to take people out on the river in open canoes so we could give it a go. If there was enough interest you might want to look at getting an L3 coach to toddle along with you that way you may not need the usual 2 to 3* ratio.
Sarah
Love many, trust few and always paddle your own canoe