Just an update, I finally settled for a µ-250CRS.
http://www.cloudynights.com/uploads/...4695_77088.jpg
It'll be mounted on a Vixen SXP and will probably push the the limits of the mount's payload capacity. I'll still need tube rings & a mounting plate; will probably look at something like Parallax Instruments for that if their rings are lighter than Takahashi's. If there's any other recommendations on light (and cheaper) tube rings than Takahashi's do feel free to recommend.