Hi Ben,
Process Explorer only verifies the signature of the image on disk, not in memory. As these signatures are not generated on a per page basis, it is not possible to use them to verify code in memory.
I plan on releasing a prototype plugin that validates in memory code on Windows next week, alongside my presentation at DFRWS. This is not achieved using the existing digital signatures however, but with hashes built from Windows executables.
I hope this answers your question.
Regards,
Andrew White