Re: DPMS over DP->DVI adapter not working
Looks like this really was a driver issue; I upgraded the drivers from 260.19.44 to 270.30, and that appeared to have solved the problem. The monitor connected over the DP->DVI dongle is now suspending correctly.
Attaching a bug-report with 270.30 running (i.e. the working case), in case you want to look for differences.