Surely they're missing a trick here - if the TCAS system can tell the pilots of affected aircraft, it shouldn't be too hard to have it notify ATC. Of course, cost of modification is probably the limit, especially if there's not much evidence of accidents caused by TCAS RA and ATC conflicting. I can only think of one offhand, but that was ultimately a pilot training issue where the pilot followed ATC when instructions from the TCAS said something different. Adapting the squawk would be an obvious mod, then there would be immediate visual on the ATC radar that an aircraft was doing its own thing. However, $$$ is always going to be the issue.