Someone coming in for an overhead join (might be a non-radio aircraft) might be concentrating on sorting out their circuit pattern and planning the descent onto the deadside....
I would hope that somebody on an overhead join would actually be in the ATZ, while people on an overhead transit would be above. Or, at least, somebody transiting overhead would not do so at the overhead join altitude.
As far as I'm concerned, biggest danger would be people doing airwork (aerobatics) in the overhead, or other people using the airfield as an en-route waypoint.
After some of the incidents I have seen with aircraft other than the competitors pratting around in and through the box, I think all of you competitors should have your aircraft fitted with some sort of paint spray gun capable of giving a good coating of something in a nice lurid shade to any box infringers. Imagine the fun they would have trying to explain the nice new paint scheme to the CFI or CO.
Actually, a lot of aerobatic aircraft are fitted with smoke systems, but are not allowed to use them during the competition. But a few bursts of smoke every now and then would be visible for miles around and would serve as a reminder to the blissfully ignorant that something might be going on.
Whether that would be reason to stay away, or to go and investigate these funny shaped clouds up close and personal I don't know though.
So say for instance I was receiving a LARS service, I'd just leave that frequency temporarily to talk to the airfield concerned then change back after transit?
Yes but do make sure that the LARS provider knows you're going to be leaving for a few minutes.
If you have two COM boxes, you could also listen out on the airfield frequency on COM2, while staying with the LARS provider on COM1. Listening out on two frequencies at once does increase your workload though.