I think it would be fairly easy to add a custom list similar to how (custom) genre works in the app through the use of ini file.
In this particular instance I use genre and composer most often (my library consists of around 50-100k tracks (whether you count multitrack files separately). The developer could place an ini file similar to genre.ini mapped to customfield/fieldname.ini or whatever and if it exists load that data. Yes I know that it seems somewhat trivial but I have 864 (dedicated ie. not counting compilation albums) folders which correspond to roughly that many composers. If we break away from classical music this amount could be insane.
Beethoven Ludvig van blah blah=
Mozart, Leopold (no one cares)=
How much use could we get out of this expansion? Little perhaps, but it would be super convenient in lieu of a history of inputs for that field (which could be quite memory intensive depending on the size of the field). Thanks for listening.