Junction box can be mounted under the stem, that's how I have mine, but my question would be if they have a long enough cable to reach all the way to the stem if being mounted on the brake levers. Note that they are designed to be mounted on the tops, near the stem and be cabled either to the Di2 brifters or to the A junction at the stem. Both only require very short cables.

[MENTION=5802]sch[/MENTION] - Is there anything in the e-tube app when configuring via bluetooth, or when connecting via USB that would allow you to change button functionality? The brifters and sprint shifters can be configured any way you want short of mimicking SRAM's "hit both sides to shift FD" setting (which I'd really like to try out). I'd be slightly surprised if you can't configure the R600 to do the same. That being said... given Shimano's strict regulation of what chainrings or cassettes you can have to do synchro or semi-synchro shifting... I wouldn't be extremely surprised.