I see no issues with extending a 15A circuit with #12. As noted, it's common for dealing with voltage drop.
Originally Posted by pdh
What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?
No, because the neutral is oversized in those cases for overload. The ground is more for fault clearing/shock protection, and is never going to see the extended overload like the neutral serving switched power supplies could.