How would you suggest going about doing that?

Without the proposed option, the alternative is leaving the build process vulnerable to environment variables potentially breaking the build process by patching configure.ac as follows:
-PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
+PYTHON_FOR_BUILD='PYTHONHASHSEED=0 ./$(BUILDPYTHON) -E'

Otherwise all environment variables affecting the Python interpreter would need to be cleared in addition to setting PYTHONHASHSEED=0.

Without these `hacks`, making build outputs to be deterministic means fixing marshal to essentially sort elements when dumping unordered objects. Would you rather see a patch going in that direction?