I'd think you better ask on Q&A to be sure. It may violate T21, as it might not really be considered part of your OPERATOR CONSOLE (per the glossary, the purpose of which is to relay commands to the ROBOT), and so if it's not allowed per T21-A, and not allowed as a strategy device per T21-E due to attaching to another ALLIANCE member, it wouldn't be allowed. It's also possible somebody might not like the safety concerns of wires running between people who might be moving around a bit behind the glass or that people might not be able to hear field instructions with a headset on, but I doubt it'd actually be disallowed on those grounds.
Per T21-B, non-powered signaling devices are allowed, so I think the old-fashioned tin-can-and-string approach would be fine.
