My plan for this didn't work out so well, as we have packages like media-sound/easytag which actually do a round-trip from C and C++. I knew it was a hack (as I said in 5b0d1e373dcd8c4f79735d033d205a6f36698c59) but it didn't hold up particularly long. Will revisit it tomorrow or so.
For the record, emerging media-libs/id3lib-3.8.3-r12 then media-sound/easytag-2.4.3-r7 worked for me. I hit this independently, and opened a bug about it on Easytag: https://gitlab.gnome.org/GNOME/easytag/-/issues/109