The 'only' other change recently was a monitor brightness tool from Entech called mControl that uses DDC/CI to alter the monitors' internals (hardware stuff, but no drivers installed as far as I can see). Of course I suspected that most, but it was installed Monday, and I upgraded WinSCP Thursday (yesterday), and mControl wasn't loaded for at least the last crash. BSODs extremely rare these days.
Win XP. I was not actively using WinSCP at the time of the BSODs (although had been minutes beforehand in at least 2 cases, quite likely all 3). At least 2 were while using a Guest account over RDP (it freezes, I think "oh come on hurry up network", then go into the next room and see the BSOD).
My next step was going to be to disable drag and drop (DragExt.dll) but I have just uninstalled and downgraded as above. If you're extremely interested in replication I could do it again but I'm not too keen on BSODs, and it takes a few hours to crash.
I'm not overly worried about this but thought I should report it, just in case it helps solve a problem in general.
If it keeps happening after the downgrade/revert I'll post a followup to mark this as a false positive.