Thanks for the input folks. The posts to date have led to interesting reading on alternative rig types and trying to make sense of a lot of photos of different canoe-sailing setups.
I enquired about going to the Windermere OCSG meet on 24th/25th Oct to see some rigs in action... only to find myself, yet again, back with (and ever more impressed by the helpfulness of) Dave Stubbs of Solway Dory... and back where I started, thinking that I should just bite the bullet (and increase the mortgage) and get a pair of Solway Dory bermudan rigs!
For my solo purposes I am now finding their 44 sq ft bermudan rig (which apparently reefs to 30 sq ft or less without losing too much shape) most appealing. That said, the Jensen 18 was bought in large part for my daughter... and with alternative mast steps, that could be used (reefed to 30 sq ft where necessary) as a main in a ketch setup with a 14 sq ft mizzen.
With 2 crew, that set-up is apparently very manageable, and I reckon it would be a lot of fun.
My query now (please bear with me for the reasoning) is whether I'd be better off with a 22 sq ft mizzen given that I'd probably be best off reefing it to 14 sq ft before setting out if I was anticipating for force 3-4 and above.
Now to explain what I hope is NOT a crazy notion!
I'm thinking the 22 sq ft unit could be used on it's own (with a more central mast step) by by wife and daughter for solo purposes (given that neither, yet, are ready for a 44 sq ft rig)... and (crucially) could double (on it's own, again using the more central mast step) as an expedition / heavy weather rig (as the 18' Jensen could just about accomodate that mast laid flat, either above or below the thwarts, without impeding either paddler overmuch - which could be useful for SOME of what I plan to do).
Given that the 22 sq ft rig (apparently) reefs very nicely to 14 sq ft (and that reefing below 14 sq ft is unlikely to ever be needed), the only downside I see to the 22 sq ft mizzen compared to the 14 sq ft mizzen (aside from a few more pennies, and slightly more difficult stowage) would be the overhang of the boom at the stern - unless I make the rear seat removable (on the cards, but that's another story), my mizzen mast might be just 38" from the rear of the boat, with the boom perhaps being somewhere between 48" and 60".
I like the solution in theory... but you good folk have the experience: what do you think?