My best guess for now is this is an experts question to Mark...
I don't know about midi clock, but I would say Composer should be able to sent realtime clock - if not this is probably a feature worth considering.
External synths usually can sync on midi clock for things like tempo synced lfo's, arpeggiators or step sequencers and alike - so that's not only important for hw slave sequencers.
Probably the sequencer also needs midi realtime start / stop commands to notice midi clock and Composer does not sent these with pressing start/stop buttons.
I am not sure if Notation Composer can control jumps in slave devices as I guess this would need
midi time code and/or
song position pointers support to sync that.
I am curious too. If I can find out more I'll tell you.