Your copyright is in effect from "day one" of fixing its form. Registering gives you proof and a few more legal protections in the event of an action, but it does not grant copyright itself; that is a given. So I would think in the case you describe, the best answer would be to both register, and also to have some time-stamped self-held evidence as well (MIDI files, wave file or MP3 with proper notation, chord and lyric sheets postmarked to yourself, etc., etc.). Anything that could establish a date/priority for yourself in the interim while your registration is/was in process...
GJ