This link flag should only be applied to the python executable.
There is already LINKFORSHARED which is great for that.
LDFLAGS will apply it to all the shared libraries which is not really what we want.

I can compile Python with support for 512MB without any modification to configure like that:

CC=xlc_r OPT="-O2 -qmaxmem=100000" ./configure --without-computed-gotos --enable-shared
make LINKFORSHARED="-Wl,-bE:Modules/python.exp -lld -Wl,-bmaxdata:0x20000000"

It is just not very elegant and will make the builbdot script even more complex than it is currently.
Also it will make it more difficult for AIX users to find out about this limitation or build python with support for more memory (though I agree this represents very few people).

If you don't want it in configure though, I will add a note in AIX-NOTES about this limitation and how to build Python with support for more memory.