http://support.apple.com/kb/PH13383 states early that Logic X functions as master only. However, when MMC, Midi Machine Control, comes across the wire, Logic will start/track/stop based on external commands. NOT MTC, Midi Time Code, though, if I understand this support article.

it's also kind of sad that Logic doesn't support standard MIDI sync commands, where I was doing this back in the late 80's, early 90's with MOTU Performer..

IF the loop is a Software Instrument, i.e. green MIDI track, with the track editor open in either Score or Piano Roll format, you could highlight all the notes in the loop, starting where you want the chord change, and transpose-by-dragging the notes (which will all move as a group) up or down to the desired beginning note. If the loop is a blue/Real Instrument, I guess the pitch slider is all you've got. Regarding going to sevenths, there is a point beyond which the loop library can't help you, and this is one of those. You're gonna have to limber up the fingers and play..