As a desperate measure, took your advise and cleaned up the circuitry inside (and with isopropyl alcohol, no less!). Went to the extent of removing the metal shielding cans and cleaning under it. And I'm afraid the battery issue remains just the same :-(. As a nice side effect, I did end up with a N900 with shiny clean innards, though.

At this point, I'm resigned to living with the issue. And it's quite manageable, with a spare battery and and external charger.

I'm still interested in getting to know why the powertop/zzztop results vary so much.