This idea's been discussed over on iomtt.com. Couple of points on your post though...

There would be no need to remove one of the existing races from the schedule as the race program is already TWO races short as it is. I think the problem with having potentially slower riders doing a support race would be concerns about relative lap speeds in practice. Whether this is just a perceived danger or a real one I don't know but I can't see this getting off the ground.

I've had some ideas about how to fill the gap and Paul P has suggested he and I meet at the TT to kick some ideas around. It's old news that I really fancy a Twins/Triples TT and there are plenty of manufacturers that could be approached over this. The 250s are coming back up, so maybe they will soon have the potential to return to the TT.

As these classes are less likely to be populated than the current 'Super...' clones by the top names then it may give riders let down by the existing situation a chance to compete.