Thanks, everyone. Through a combination of your kind responses and some further research and cost / benefit analysis, I am - for now, at least - shelving the idea of an ANR glass sandwich
Instead, I'll be trying the Lomography Digitalizer 35 and 120 film holders / masks. Since I cut all my negatives into strips to fit standard Hama glassine sheets, these should suit my way of working. They have a pretty good reputation for keeping film flat without the need for glass (albeit there's still the potential for a tiny amount of lateral "bow" with particularly stubborn films), and the method employed for doing so makes sense. There's a ready market for these holders with both flatbed and DSLR digitising users, it seems, so if I find they don't meet my expectations, I should be able to move them on at only a small loss
One thing I'm learning from all this is that there doesn't seem to be a single negative holder that satisfies all of the following:
(1) supports a variety of formats in 35mm (e.g. standard, panorama) and 120 (6x6, 645, 67, 6x9
(2) supports both cropped and "with border" framing
(3) allows easy loading and alignment of both continuous rolls, short strips and individual frames
(4) allows easy advancing to subsequent frames
(5) has zero feeding issues on entry and/or exit portions of the holder
(6) keeps negatives completely (or very nearly) flat, including the leading / trailing edge of those at the start and end of a trimmed strip or roll
I already own two negative holders... the EFH "Essential Film Holder" and the budget-oriented Pixl-latr. The EFH is mostly great, but falls short with first and last frames of curly negatives (especially 120); the Pixl-latr keeps negatives even flatter than the EFH, including first and last frames of curly stock, but is
very fiddly in initial alignment and subsequent advancing of frames. Both are versatile in different ways, both have their uses and are good products at their respective price points, but neither is close to perfect in relation to my own requirements. The Digitaliza holders seem like they should be better suited to my needs - but we'll see. I think a comparison review might be warranted at some future juncture, once I've had a chance to try them