Ok, I delved into this and found that it's not possible. Here's what I did:
I downloaded Midi-OX and installed it on my laptop. I hooked up Midi out on the Roland to Midi-in (via a Midisport 2x2 box and USB) on my laptop. I opened Midi-OX in Monitor mode, noticed that there was a constant stream of F8-Timing Clock and FE-Active Sensing messages coming from the Roland, so I used the Midi-OX Filter to filter these out.
Then I played around with all of the Midi-related controls on the Roland. I could see Note events and other types of events, but was unable to find any combination of settings that would generate Midi Start and Stop messages when the "Play" button was pressed.
I did notice that the "Midi Ctrl" button on the Roland takes the keyboard out of whatever mode it was in and puts it into a Midi controller mode, which means that the different buttons on the keyboard will transmit different types of Midi messages. At first I thought this could be used to program the Play button, which it can, but it turns out it's only for when the keyboard is in this Midi-Ctrl mode. When you go back to Recorder mode or Play mode, those settings don't have relevance.
Then, I experimented a little with the Click-out settings (the click track). The recorder mode can be redirected to Click-Out. I thought this could be then piped into the Clock-sync port on the Drumbute, so I hooked them up with a 1/8" TRS cable. But it turns out the entire recording comes through, not just the click track. Of course the entire recording has all kinds of sounds which confuse the Drumbrute's synchronization, so this obviously didn't work.
In short, there's no way to trigger Play Start/Stop/Pause between the Juno-Gi and the Drumbrute. The only thing that you can synchronize are the midi tempo messages, which really is of no use whatsoever if you can't synchronize the start of your drum sequence with your track. I feel this is a major oversight on Roland's part, it severely limits the capabilities of the digital recorder on the Juno-Gi. Let's say I lay down a drum track from the Drumbrute. Later if I want to change the pattern I would have to manually try to sync it up.
