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I don't understand this issue at all:
a) _bootstrap does not currently use any private API of marshal. Instead, it has functions _w_long and _r_long implemented in pure Python. So where is the special functionality that only importlib has? Anybody could easily replicate these functions.
b) Isn't it easy to implement it as such:
def _w_long(x):
 return x.to_bytes(4, 'little')
As for fix_co_filename, I think it would indeed be useful if marshal.load(s) supported an optional filename= parameter, which then fills rf.current_filename. It's better to load it into the correct form in the first place instead of fixing it after loading completed - in particular since marshal already has a mechanism to update all filenames.