Did you -update- to 3.5 or did you do a new installation of 3.5? Because as I said previously, if you simply updated the data would be there.
Are you using the Store version of MB?
This line in particular implies that you did not -update- to 3.5 as 3.4 is still available.
Alternatively, if we don't know what file it was stored in, is there a way to identify in 3.4 all the "No lyrics" songs I've flagged, so I can add it to their lyrics field. And if we go that route, is there a way to make checking "No lyrics" automatically add it to the lyrics field.
It would be stored in MB's database (I've said this before also.) You can't access the database except through MB.
If you really do still have 3.4 installed then do a custom search where "Lyrics?" has no value.