Interesting problem. One thought comes to my mind, I never go into IMC, apart from for a few seconds, without some sort of radar cover. Assuming this transiting aircraft was in dense IMC he either didn't have any radar service (or had just been handed over and was free-calling), or he felt is wasn't necessary (frightening). If he is cavalier enough to report VMC when he clearly wasn't, I hope I never get anywhere near him.
IMHO this is worth CHIRPing if only to remind all pilots of basic air law and good airmanship.
LF