Since this is now over 2 years old is there any progress?
Edited to include the link to Scudo: https://github.com/llvm/llvm-project/tree/main/compiler-rt/l...
If I recall correctly from the mailing list when this was first proposed, someone had a use case for custom libc but extending it to a general purpose libc diminished properties that would distinguish it from (e.g.) musl-libc.
https://twitter.com/chandlerc1024/status/1464530620416073735
(link to paper as well as code in that tweet).