The good news is that I just managed to recover a bricked n900, the bad news is that it seemed to require access to a second one or at least a compatible battery. Ironically my n900 refused to charge in the bricked state and at the same time would not allow flashing unless the battery was charged. (stuck at the waiting for USB part)
Note that I inserted the battery while the phone was connected and the software was in the waiting state.

The simplest solution was of course to use a different battery during the charging process. You might not have the luxury of an extra device available at the moment but as soon as the Nokia care centre gets a few devices it should be fixable. I'm confident it is unbrickable though as it sounds like the exact situation I was in.