I tested your idea out, and yes, adding a "song" to say a druid's list of usable spells does allow the druid to use it, but they still cannot move while casting it. I did not try out instrument mods to the added spells, but based on my previous post, I doubt they would effect the song.
Also, adding a spell to the bard's "song" list will let them "cast" it. HOWEVER! it still lets them run around while it's being cast and not interrupt the spell due to movement thus making bards the UBER class to be if you did a true all/all spell/song server.
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