It sounds very similar to what RPython and Pypy do. You can even use RPython to create a VM for a language besides Python. I read a good blog post from an author who had done that 5 years ago.
Given how HolyJIT is implemented (with compiler plugins etc), I believe that it does this work at compile time, yes. I haven't actually confirmed this though.
The goal is to optimize the code both at compile-time and at run-time. We need to optimize it at compile time to prevent slow start-up, and we need to optimize it at run-time to benefit from profile guided optimizations.
