@Ross

> The intention appears to be that the sysconfig.py in the build is used.  In my case that directory is also full of shared libraries that Python happily loads, and then fails.

No, the intention is that the native python finds the sysconfigdata module named $_PYTHON_SYSCONFIGDATA_NAME that is located in the directory where the extensions modules (.so libraries) have been cross-compiled. $_PYTHON_SYSCONFIGDATA_NAME is a pure python module, so it can be loaded by the native python.

Please note that in your first post PYTHONPATH is missing the first part, i.e. the path to this directory obtained with:
    $(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`:)

When PYTHON_FOR_BUILD runs, it does not load any extension module so the Illegal instruction may be caused by some other problem.