Seems that's one way to make those symbols public. Nice hack - though if you ever need to access a different private symbol, you'll have to rebuild perl again.
I'm curious to know whether the Devel::Cover that you built with this hacked perl-5.10.0 will work on an unhacked perl-5.10.0. (Others would probably know the answer to that without needing to test it.) Have you tried that out ? If it works on the unhacked version of perl, then it could be handy for others if the binary of your build of Devel::Cover (either ppm package or just a tar.gz of blib) was made available.