EastCoaster - I think the problem here is with the term "centre-fix" itself which may be compromising unambiguous understanding between ATC and aircrew.
It's my opinion that a "centre-fix" is construed by the crew as not just a routeing point or a possible holding point but as part of the approach ie they would expect to cross the centre-fix at 2500 or 3000ft or suchlike and then turn onto the localiser.
Therefore if the pilot requests to route to the "centre-fix", I would say that he is asking for a "no-delay" approach. If permission is granted then the crew would be justified in preparing for exactly that.
You say that the instruction from your end always uses the name of the published position. Perhaps you should maybe emphasise that the pilot is not being cleared to the centre-fix with a "negative route to XYZ expect no delay" or something similar.
Are we anywhere near resolving this?