[Mark McLeod]

FIRSTSearch is undergoing a server change so it's not currently available, but I found this after a quick search of the Q&A. I remember more extensive rules disallowing any form of communication between the drive team and anyone off the field, be it signs held up by the crowd or telecommunications.
Standard disclaimer regarding past rules not applying to future games.

In any event the purpose of the game rules is not to imagine and categorically list everything you should not do, but to give you guidelines of FIRST's intent and leave it to your developing good judgment to know what's acceptable and what is simply skirting the rules.
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ID:1247 Section:7.12 Status:Answered Date Answered:1/19/2005

Q:It's clear walkie-talkies and two-way radios are not allowed. Is there any communication allowed between the COACH and someone in the stands, say, with a cell phone, during a match?
A:No.
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ID:1560 Section:7.12 Status:Answered Date Answered:2/9/2005

Q:Will wireless networking be allowed at competitions?

A:Yes in accordance to the statement that follows below, BUT IN NO CIRCUMSTANCES is communication to people or devices outside of the driver station / Team Zone allowed. ...