I believe it would beneficial for all datapack creators if there was some way to implement custom TMs and TRs. Mentioned in another TM/TR post by Novoro, there's already a “blankimprintrate.json” which could work for such a datafolder.
If adding custom moves to existing TM datafolders may screw up the generational system for TMs - as that's being used to organize everything - perhaps a specific section for custom Tms/TRs could be created so that custom moves don't get cluttered in with pre-existing ones; possibly named "tmmovesCustom"/"trmovesCustom". This would function like how you would add custom tutor moves for naturally-spawned move tutors; add in the move you wish and in the game a TM or TR will be created.