not sure about the parameters (can't find such a message in HLSDK or Quake1 sources either), however according to pingfaker such messages can't be intercepted within pfnMessageBegin() with metamod, as it's sent in the Engine instead of game dll. What pingfaker does seems to be sending such messages to "override" the same messages sent by Engine.
btw, maybe you can get better answers for HLSDK-related problems at the
hlcoders mailing list.