Cougar,
I don't see what your problem with the call is. I had this happen the other day. Leaving controlled airspace on descent into busy CTAF. We made an all stations broadcast on centre and got several replies from VFR traffic that were transiting the area, but weren't on the CTAF as they didn't need to be. The traffic replied to our call, and from there we arranged separation and that was that. Had we not made the call, we probably wouldn't have known about the traffic until we got a traffic advisory on TCAS, and that's assuming they had a transponder! And as for centre seeing us on radar, radar coverage is marginal at low level in this particular area, and once again it depends on whether others have their transponder on
