I've been trying out a technique several people have told me to try, one which I haven't managed to get working in the past. For some reason, it now seems to work - don't know what I was doing wrong before, but at least it's another technique to add to the arsenal.
It involves boiling water, and dipping the warped part of the print in for a few seconds. Then bend it to shape (with something heatproof) and hold until it cools down again. It's not perfect, you do end up with a slightly wavy surface, but it can make warped parts far less obvious. The buffers on this SER brake had a distinct upwards angle to them, but it now seems a lot better.
I'm struggling to work out why this works though - the resin is cured, not melted, so I don't know why heat would soften it again. One theory is that the resin is actually a composite containing tiny thermoplastic particles, or maybe that it breaks down the cross-links just enough to enable it to move. That will remain a mystery!