[thr0w4w4y1337]

awnumar/memguard[1] exists and does even more

1) allocations via memguard bypass gc entirely

2) they are encrypted at all times when not unsealed

3) pages are mprotected to prevent leakage via swap

4) and so on...

Not as ergonomic as OP's proposal, of course.

[1] https://github.com/awnumar/memguard