Time to update with my solution. Given that I was using a mono CCD, I really needed to find a satisfactory way to change my filters (LRGB and NB). I ended with with the following imaging train:
-- QHY22 (15.5 mm back focus)
-- 0.5 mm
brass spacer ring
--
TS Filter slider (15 mm optical length)
-- 2 + 1 mm brass spacer rings
--
TS EOS-EF to T2 adapter (10 mm optical length)
-- Total optical length = 44 mm
The optical length matches exactly with the FFD of the Canon EF mount lenses, and this setup allows me to change filters on the fly without unscrewing anything.
Attached is an image of the setup (connected to a 135 mm f/2.8 EF prime lens).
I'll be mounting it on the top dovetail of my SW120 refractor (which will be used as the guidescope), with two DVR125 delrin-tipped guidescope rings used to secure the camera body and the lens.
Unfortunately, it's been raining/cloudy here in Tasmania for the past fortnight, so I've not yet had a chance to image with it. But when I do, I'll post back with the results.