I read again some remarks about alignement, it was suggested to provide allocators providing an address aligned to a requested alignement. This topic was already discussed in #18835.
If Python doesn't provide such memory allocators, it was suggested to provide a "trace" function which can be called on the result of a successful allocator to "trace" an allocation (and a similar function for free). But this is very different from the design of the PEP 445 (new malloc API). Basically, it requires to rewrite the PEP 445.