While I agree NetBSD needs to support and work well with GPT in order to interoperate with other systems, there is also prior art in extending disklabel to 64bits - OpenBSD did this back in 2007 (though there were a fair few follow up commits to cleanup the fallout :)
https://github.com/openbsd/src/commit/ddfcbf38c8ab6225a6b172d829aa957007d2587f#diff-192d23728acf9d8a70ab7259784d4162 David
