After a fair bit of digging, I still failed to find any designated Blue Badge bays.
However, according to Bristol CC, Blue Badge holders can park for unlimited periods in on street Pay and Display spaces, for free. I'm less bothered about the 'For Free' bit than the actual distances involved... I can actually walk considerable distances.. sometimes.. It's all a bit unpredictable with two metal knees and a 'dicky ticker', but there certainly seems to be a limit to the total walking, standing etc, I can manage in any one day so turning up to a show already knackered is a bit pointless.
However, I did find a Bristol CC 'Pinpoint Information' map, which shows a multitude of different Council assets etc., including On Street Pay and Display Parking machines, indicated by little blue dots. That seems to indicate that there's plenty of On Street P&D parking in the city, especially just north and just east of the venue. I'll risk that and if that fails I'll get into a Multi Story.