Threemiles...
When I allocate a squawk as a Controller I am responsible for checking that the code has been correctly selected and that the Mode C response (if available) is within tolerance + 200ft. In which case the data can then be used with confidence. You correctly say that TCAS uses altitude comparison of Mode C data... but traffic that departs from a non-radar/ATC environment and selects a conspicuity squawk (7000) has never had the data verified... i.e. the data picked up by another ACAS system is unreliable?... an yes we do see SSR that is out of tolerance!
I did not advocate that pilot's should receive the Mode A.., for although in the case I use the pilot might have recognised conspicuity, it would be unreasonable to expect them to be able to decode every code in-flight and then know which are verified and which aren't... but, I'm suggesting maybe it's not the wisest decision to continue towards a confliction, in IMC, relying on ACAS to prevent collision when the in-cockpit information might be inaccurate?
In addition to the 7000 squawk, many special purpose codes e.g. 0020 - Air Ambulance, 0033 - Parachuting, 0023 - SAR Ops, 0036 - Police, 0047 - Fishery Protection etc etc are considered as unverified data
I hope that makes my original question clearer (and that I don't get shed loads of replies about not providing ATS in unregulated airspace... which is another issue altogether?)