I don't understand... everyone I've ever seen arriving in a wheelchair in HKG is provided with a wheelchair escort, which the airline requests in advance. 90 minutes should be plenty of time unless the porter/escort doesn't show up.

As an aside, I have completed well over 100 trips on CX in F class and have never been booted out of my seat, nor have I ever seen or heard of this happening to anyone else (last minute change of equipment excepted, in which case a CX rep met me at the counter and handed several hundred U.S. dollars in cash, which more than compensated me for the fare difference between F and J).