So I had a tinker with the various bits and bobs I have, to see if I could extend the light path of the right-angle port to get an eyepiece to be more-or-less par-focal with the DSLR body on the straight-through port.
First I tried my "Shorty Barlow" tube (with the Barlow lens element removed), but it was too short.
Then I tried a 45-degree "erect image" diagonal - but it was too long.
Then I tried a simple 90-degree star diagonal - and it was just right!
It's not very elegant - but where there's a will, there's a way!
The eyepiece focus is not quite spot-on when the camera is pre-focussed, but it is perfectly adequate for target-finding and framing, using an eyepiece in the right-angle port, and then flipping to the pre-focussed camera on the back port. (I imagine I could improve things somewhat with an adjustable extension tube for the eyepiece, but this set-up will do for now.)
The field of view in a 25mm Plossl is about the same as the camera gives, and my 40mm Plossl gives a slightly wider view than the camera, so is probably the best choice for target finding.
Skies in Brisbane today are crystal-clear, so I'll have a go at some night shots tonight (as long as the fog stays away!)