All right, status report :

To my great surprise, it worked perfectly at first try :-)
Thanks again Oblomow for the detailed instructions !

After installing kp49 and h-e-n, I connected the N900's usb output to my DAC's input.

After the three-step h-e-n setup the DAC's LED turned to green, and cat /proc/asound/cards showed a detailed description of it.

Then the magic command (mplayer FILE.mp3 -ao alsa:device=hw=1.0) played everything through the DAC, yay ! :-)

So now we have a definite proof of concept that it is doable.

What's left is about usability on a day-to-day, on-the-go basis :

* a script that automates h-e-n speed/VBus/enumerate

* redirection of the standard sound output to USB so that any music player can be used

* buy a new battery, because h-e-n seems to suck down fast, although the DAC has its own power :-)

Getting really close now, thanks to you !