I've looked into criu. It's usable, but not as good as unexec and not yet properly supported from packagers. Even if only the build step needs it. And it doesn't produce a single binary, which is ok I guess for emacs.

Easier IMHO is to keep maintaining a proper malloc implementation. glibc were not able to update to ptmalloc3 anyway, and now they want to destroy ptmalloc2 even more. I wouldn't trust them.