Comment #6 on issue 2783 by [email protected]: wrong placement of timesignature http://code.google.com/p/lilypond/issues/detail?id=2783
\override Staff.TimeSignature #'Y-offset = 1works just fine for juggling the vertical position of signatures around. So I'd say we just use the 0 position for the time signature, and leave the rest to the user. If he is defining irregular barlines, he'll know best where he wants the signature to end up.
