Wow, you are hard core!
Having owned a A-H Sprite as a daily driver (and used in the snow every winter), I think that the se7en is even less user friendly than your MG (tho not your bike). With the top up it is very hard to get into (remember, no doors, just side screens. So it is a bit like climbing thru an open car window). And you have a lockable trunk. In my se7en, if I stop somewhere, my computer, briefcase, etc., all has to come into the store/bar, etc with me, it cant be left in the vehicle.
With everything buttoned up, my Caterham was reasonably dry in the rain. But because it was soo low, it tended to really get drowned in puddles, trailer backwash, splash etc.
I suspect a well maintained se7en is at least as reliable, probably more reliable, than your MG.
Another consideration, you can drive an MG anonymously. In a se7en, EVERYONE's your friend: what is it, how fast does it go, is it a V8, did you build it?? If you stop at a gas station, traffic light, grocery store you are surrounded by people who are fascinated and want to talk. In small does it's fun, as a way of life it is a huge PITA.
Based on your supermoto experience, I suspect you cld make a se7en work. If you do, you mite be the only person on this site who has tried it. Or am I wrong? Anyone else doing a se7en as a DD? Closest I got was summers in my '97 Caterham, with no passengers, on dry days.