I was experimenting today a little bit and got one step further...
I was connecting the usb mixer (yamaha mw10c) to the n900 (with kp49), enabled host mode via h-e-n and entered "sudo modprobe snd-usb-audio" in a shell. Afterwards I started Audacity through Easy Debian's LXDE. Then in Edit->Preferences->Devices usb audio devices show up. The good news: recording seems to work, at least sometimes with sluggish performance, but I got something that sounds quite ok after exporting to a wave file and playing in another program. Which leads to the bad news: until now every attempt to play via usb leads to a frozen screen, beeping noise (like some binary garbarge) playing through the usb audio device and after some seconds a reboot. I haven't spent much time with this upto now, so there might just be something wrong with sample rate or format or whatever. Also Audacity is really slow when playing back through the default (n900) device, with lots of hiccups. But I will keep on trying, maybe I'll find some setting that actually works.

BTW, thanks to the authors of kernel-power and h-e-n for their great work!