For anybody who stumbles upon this, I took two approaches... maybe three.
1 -- I rented a demo-hammer (two-handed electrical device, 10a, excellent when taking tile off subfloor). That approach lasted ten minutes. The hammer I was using wouldn't activate without sufficient pressure on the bit, and I could only get over that threshold by pointing the hammer relatively straight into the wall -- risking the underlying cinder-block wall. It was also heavy and loud, which pissed off my neighbour (semi-detached house).
2 -- I used a 7" grinder with a concrete-grinding cup. This worked well, though obviously generated insane levels of dust. The dust is very fine, which clogged the filter bags very quickly. No-name bags would clog and then burst, clogging the filter; name-brand bags would clog and lose flow but at least they didn't burst. Once they lost suction, most of the dust remained in the air near the tool. Eventually I gave up on the vacuum and worked blind. I relied on feel and sound to know when I was grinding gypsum and when I was merely smoothing the cinder-block. I did approximately 20m2 myself, then hired a contractor while I went to my day-job.
3 -- When the contractor wrapped up, I used a one-handed air hammer with a 3" blade-chisel, as recommended above. This was excellent where gypsum remained, which was only at the edges. It was the best because I could get under the wire mesh, instead of grinding through and showering myself in shards and molten droplets.
All three methods created a lot of noise.
The grinding created a lot of dust, and eventually the dust was sent out the window with a series of fans. This was good for the first day, but on the second day someone mistook the clouds of white dust for smoke! I had departed for the night, so five fire trucks attended before someone told them where to find the trades key. After that, I called the FD dispatch desk to warn them I would be grinding.
I hope nobody else ever faces this issue -- much easier to just tear the house down. :)