I'm planning to review some of the other patches from the current patchset soon.
I've looked into the patch 0003.
The patch looks in good shape and is uncontroversial to me. Making memory structures to be dynamically allocated is simple enough and it allows to store complex data like lists etc. I consider places like this that expect memory structures to be flat and allocated at once are because the was no need in more complex ones previously. If there is a need for them, I think they could be added without much doubt, provided the simplicity of the change.
here I suggest adding Assert(rel->rd_amcache == NULL) (or maybe better an error report) after calling free_rd_amcache to be sure the custom implementation has done what it should do.
Also, I think some brief documentation about writing this custom method is quite relevant maybe based on already existing comments in the code.