This made me realize we may already have such a setup, sort of: my wife/stoker and I have cellphones with bluetooth headsets, and on Verizon's "family plan," all calls betwixt our phones are free -- seems like this ought to work as an intercom, we'll try it this weekend....

(Obviously this would be an absurdly expensive solution to teams not already having this stuff, but since we do it's free, in the sense the costs are already sunk.)