At the last print shop I worked at there was a trick while imposing a booklet that allowed us to print booklets that weren't in multiples of 4, where "half a section" (maybe not the correct technical term) was chopped off to allow for, say, a 42-page booklet. I can't remember how it was done but is anyone familiar with this? No information on Google.
What you are looking for is called Perfect Bound.
This is really just a series of saddle stitched booklets each of which is read in sequence.
For example, the first booklet may have pages 1–32, the next 33–64 and so on.
You generally select the size of each signature; 32 in the above example.
If the last signature if it is not full length you can just pad it out to full length, or just to a multiple of four pages.
The Quite Imposing Plus Plugin can do this easily through Acrobat.
(I'm not affiliated with Quite Imposing Plus)