Well, I don't think anything that would inhibit a pilot being able to transmit would ever pass muster, simply because if something broke it may keep the pilot from being able to transmit entirely. You'd just be trading off what is in 99% of cases an annoyance to a potential safety issue.
To answer a couple of previous questions....
Yes, if I transmit, it overrides the repeated audio on the other frequency. That's why I really wish they'd figure out how to make it work on our low altitude frequencies where in my opinion it is needed most.
Don't confuse cross coupling with frequencies being combined. Now, terminal facilities in the US do use a different voice switching system (gotta love the government!) and I think they can cross couple across more than two frequencies at a time. However, there are only 6 sectors in my area (ZME has six areas, four with six sectors, two with seven), and only under unusual circumstances would all six frequencies be combined; typically on the mid shifts after the FedEx arrivals we go down to two scopes in my area, one works high/ultra high, the other low. So one guy has four frequencies at most, working in two cross coupled pairs. So just from a practical standpoint in a US Enroute facility, there really wouldn't be any need to cross couple more than maybe 3-4 frequencies at once.