See
p.119 of the manual. There is an NRPN control that will toggle the on/off button found on the front panel.
If you also have Key Sync OFF in the arpeggiator settings, the arp will start on the next clock (internal BPM or MIDI, at the specified note value) after a note is triggered (by keys or by incoming MIDI from a sequencer or other MIDI device). For this to work reliably, the NRPN must be sent in advance of when you want the arp to trigger. You will have to experiment to see how far in advance is required, start with a bar (1/1) in advance and then try ratcheting down through 1/2, 1/4, 1/8, etc.
Turning the arpeggiator off again will not work as reliably, since the timing of the NRPN message relative to the clock is not guaranteed. But it will stop the arpeggiator on the next appropriate clock edge. Again, experiment and see how far in advance you need to send the message without either cutting notes off way too soon or way too late.
Ableton Live does not support NRPNs natively, but if you have Max4Live, there is a Max device called
NRPN Gen 2 that can do it. If you do not have Max4Live, you will have to break the NRPN message down into it's components which are nothing but a string of CCs.
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