Merry Christmas USAflyer
I would love to say it will be fine - and I suspect on a quiet day (mid week in January) it would be fine - however from what I've seen a 90 minutes connection is too short. In some situations it would be a piece of cake, particularly if both flights operated via T5B, which is entirely possible. But intuitively I feel you want to go north of two hours, unfortunately. I guess there are two other factors here. If the wheelchair passenger has a companion who could just about manage without assistance and very light hand baggage then that will definitely help (you would get some priority at security); The other factor would be if for example there was a 90 minute connection and 4 hour connection option for the next potential flight - and it wouldn't be the end of the world if you ended up on the later service. In that case I think so long as the passengers are laid back about it, then going for the 90 minute connection isn't such a bad idea.
I appreciate I am semi-advocating lower performance standards for wheelchair passengers, which to say the least isn't a great look, but there is a long way we have to go for true equality for the less able bodied. Having said that I'm of the impression that in the last 3 months or so there has been some improvements for passengers with reduced mobility and I've even recently shared a flight with Frank Gardner where he was first off!