Now, when the system has been idling for a while with tohd running, a 20 second (nominally) powertop run reports a power consumption of 955 mW, whereas the same without tohd reports 70 mW. In interactive mode over WLAN I've seen figures as low as 10 mW, I don't know how reliable the measurement is.

Powertop suggests enabling Power Aware CPU scheduler, echo '1' > '/sys/devices/system/cpu/sched_mc_power_savings' - those of you who know the HW and kernel better, do you think that would be safe to do?