SET_PARAMETER is simply *wrong*. Servers are not supposed to support except for proprietary extensions (meaning we cannot use for just any random server).
Even GET_PARAMETER is hardly good, as it is not mandatory to implement either, but it is a defined way to ping (which SET_PARAMETER ain't).
In any case, the only proper way to ping a stream is RTCP feedback.
When it comes to the control session, better check OPTIONS Allow: header for GET_PARAMETER.

I really wonder how live555 can be so low-level though - smells like we are trying to be too intelligent here.