Well, I guess I can answer my own question (and for anyone else who has the same problem):
Apparently, regtlibv12.exe is part of Visual Studio 2010 (contrary to what I read on various Microsoft forums), but it is located in the Windows\Microsoft.NET\Framework\v4.0.30139 folder (not the v2.0.50727 folder). Using that executable I was able to successfully register the legacy typelib (.tlb).