A discussion with Ken Eichenbaum inspired a thought for better structuring Opener's rebids after he opens three of a major (in first or second seat) and hears partner bid 4C as a general slam invite. The structure:
4D = Honor cue (Ace or King) in diamonds. Might have shortness also. Responder can bid four of the other major to ask for shortness, if any exists. Opener can show the shortness after a signoff anyway if he has "extras."
4OM = Honor cue. 4NT asks for shortness, if any. Persisting after signoff again shows shortness and extras.
4NT = Honor cue in clubs, shortness in other major.
5C = Honor cue in clubs.
5D = Honor cue in clubs, shortness in diamonds.
If Opener has no honor cues (repeats major), Responder can ask for shortness by bidding +1 (4NT after 4S, or 4S after 4H).