SUCCESS! (partly)
Forcing active mode in filezilla makes it connect correctly.

pulling files from the device works at around 600 -700 kb/s. cpu usage on the n900 quietens down after a few seconds, so that doesn't seem to be the limiting factor.

pushing onto the n900 only happens at 60 kb/s.

so it works. file/directory names with spaces are a dealbreaker so far. and the speeds need improving. but it works