Andy, in brief, I worked out that I can actually do it all with Canon EF lenses, which have the advantage over the Pentax Takumars in that they are still being made! (Note: if you go the Pentax route, you will still need a M42x1.0 to M42x0.75 thread adapter).
I have on my list to initially get a Canon-EF 40 and 135 mm lens, which bridges the gap to my 384mm ED80T (with FR). I'm also considering an 85mm to fit in between, and keeping my eye out for a wide-angle lens in the 14-28mm range (must be EF, not EF-S or EF-M).
Anyhow, it works out like this:
-- CCD = 15.5 mm backfocus (I'm using a QHY22)
-- TS-Canon-EF-to-T2 adapter, short = 10mm
-- TS Filter slider = 15mm
Total backfocus = 40.5mm
With a 3.5mm spacer (I have a 3mm and a 0.5mm ring, or I might find that the filter adds about 0.5mm BF itself), I have the perfect match to the Canon-EF FFD of 44 mm.
I can also add a Nikon-F to Canon-EF adapter to this system and still be within the 46.5mm FFD of the Nikon, so that gives even more flexibility with possible lenses.
At least this is the theory
I need to wait until TS restock the Canon-EF-to-T2 adapter before I can actually prove it all out!