Hi Peter
You raised the possibility of a Skywatcher scope. My Skywatcher optics seem to be seeing limited, just like the much more expensive ones, so optical quality is probably not really an issue - once you get to the seeing limit, better optics is nugatory for DSO imaging. What you will/should get with the high end scopes is much better mechanical and thermal stability, possibly the correction of primary aberrations over a wider field, plus everything should fit together and work straight out of the box. The Skywatcher could be a remarkably good scope for wide field imaging with a coma corrector and big pixels, but I would not recommend it for high res imaging with small pixels on your high end mount unless you are prepared to spend a fair bit of time fixing the weaker parts of the design (focuser, secondary mechanics etc).
Suggest that you use a fast scope with the Trius to get the best out of it. A fast Newtonian makes sense, but as you say, there is not much back focus to play with from the coma correctors in wide use. The one that does provide a fair bit is the RCC1 (91.5mm) - it is low end (so build quality might possibly be a bit variable?), but it seems to work well over a small field and might be something to consider if you needed lots of back focus and were prepared to assemble you own system (see above comments).
In any case, a guidescope might be OK with the Trius, since that camera has low enough read noise to image effectively with 2-3 minute broadband subs at f4 - you will probably be able to get a guidescope solution working well with such short subs - unguided may even be a possibility.
"what to put on an MEII" is not a bad problem to have though
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regards ray