FWIW I got this working perfectly last night, using MIDI notes in one clip (i call the source) directed through the IAC bus to transpose another track, i call the destination. Couple of tips for other newbies like me:
* in MIDI preferences, make sure Track and Remote are ON for the IAC input, but only activate Track on the IAC output. I had Remote selected on both and it was causing all sorts of weird things to happen, including crashing Live.
* Turn off "select on launch" in the Record/Warp/Launch preference pane. Otherwise the incoming midi notes will cause Live to switch to the destination part every time it triggers.
* I tried using MIDI offset to shift the source part ahead in time so it would catch the first loop of the destination, but was finding it still wouldn't transpose. I tried shifting the loop points within the source part also to no avail. What finally worked was when I had a lightbulb moment and just shifted the actual MIDI notes in the part to the left by one tick on the grid.. voila. MIDI notes in one part, transposing another, each and every time.
Hope one day this saves someone the hours and hours it took me to figure that out.