Definitely tricky. I solved it with a Python implementation by building up a big jumpbuffer so that the moment I leave Python-land I copy from temporary buffers to the right addresses and then ultimately jump at the entry point of the newly loaded binary. It's tricky and took quite some debugging to get right, but it's proven rather solid now.

See https://github.com/anvilsecure/ulexecve/blob/main/ulexecve.p... for details. Especially the `CodeGenerator` classes with implementations in x86, x86-64 and aarch64.