Its certainly not stalled, it's just not moving at the pace they wanted. There's currently an awful lot of work going on, slowing down for winter and due to our recent poor weather, but overall they're still fairly on track. I've spoken to staff up there relatively recently and the majority of visitors they get are local to the Bristol area, plus there's a large advertising campaign on buses in the region as well, so by the sound of things they're doing alright for foot traffic and therefore funds. I'd be surprised if they hadn't factored in the Clifton NIMBYs into their plans, given they were always grumbling about the zoo (and yet can't stand the idea of it being gone, weird that).
I genuinely think the most important thing for BZP in the short-term is not to fast track construction but to get the bus services running directly to the zoo - this will also open up the student sector from the 4 local universities.
I probably shouldn't be sharing this but given it didn't go anywhere I may as well, but we tried to get a conservation project with BZP sorted at Bath University last year, and the only issue (and quite a major one at that) was transport - Cribbs is a pain to get to and the area in general is in desperate need of a loop service connecting the city centre to Cribb's, Filton and potentially Temple Meads.