Well I feel (predictably) silly - it BSODd again today, downgraded version of WinSCP and no drag and drop dll. However I had recently been using it, again.
I have enabled minidumps (full memory dumps weren't completing and/or something installed itself over VS10 breaking the debugger so I can't see what caused the crash), however my suspicion has turned to something else: Took me a while to dig back to what I was doing at the time, it turns out minutes before the first BSOD, I changed my NIC TCP/IP DNS settings: primary to an external nameserver, secondary to my internal router proxy (rather than just the router as it has been for years). Migrating an old server so my local machine is seeing lots of DNS weirdness especially WinSCP as it was hitting multiple partly-resolved domains in a row, so maybe there's something the driver can't handle about that. This is a Gigabyte GA-P55-UD4P mobo with Realtek NIC, I know some versions of the driver have bluescreened on me before, but (I thought) the one I've been running for years was stable).
So quite possibly closer to the facts there, sounds more likely than a presumably non-kernel dll, I certainly wouldn't blame WinSCP as the cause.