I've little doubt that Wg Cdr D'Oliveira had a hand in the derivation of the ASCOT callsign, but I suspect that he was by then on the staff at Upavon, at the time when Transport Command became Air Support Command (1 August 1967). Significantly, this was also the period during which management comms for the Air Transport Force were brought up to date, and the Flight Watch system was introduced. This involved HQ ASC buying into contracts with global civilian signals agencies and, as a consequence, the Command had to apply to ICAO to register a company callsign, ASCOT, together with a two-letter designator, RR.
And for what it's worth, my recollection was that the T stood for Task. Also, whatever happens now, to begin with and for many years afterwards, the ASCOT callsign would only have been used with a 4-digit task number issued via Upavon.
Before all this, we used the RAFAIR MABCD type of callsign for route tasks, but that's all a long time ago now!