One reasonably easy possibility would be to add an option to regrtest to exit with a non-zero return code if there are unexpected skips.  This might not cover 100% of Mathias's cases (but it might), and the configurability would be limited to changing the expected skip lists in regrtest.  But it would be pretty easy to do, if all of the buildbots can be required to build all of the expected extensions.  I'm not sure that this last requirement is currently met by all the buidbots.