Comment by roryirvine

That sparked a memory of rewritefs, which I used when rationalising a few hundred thousand legacy mod_rewrite rules at a web hosting company in the early 2010s: https://github.com/sloonz/rewritefs

Looks like it was based on a mid-2000s system called libetc, which did a similar job but had to be LD_PRELOADed: https://ordiluc.net/fs/libetc/