@Estel: I will top that, I'm 100% sure it will work :-)

@ineo : your use case is what I've been wanting to do all along (except that I'd hesitate between an E17 and the iBasso D-Zero, but that's irrelevant).
Everything that was said before applies : your E17 will be detected by h-e-n, just like my desktop DAC/amp and Estel's external sound card, as an ALSA device.

Streaming MP3/FLAC using software that can directly address that ALSA device is easy (using mplayer, MOC or smplayer as mentioned) but not seamless or very user-friendly.

The "way to select the E17 as the output sound device", at the system level and with Hildon apps that only use pulseaudio, is our biggest and still current problem. Nobody here seems to know how that thing really works (nobody interested in our project, at least :-)

I am beginning to fear that the problem will be solved on the Android side before we've made any progress on this. Some versions of Cyanogenmod 7 and 9 already enable USB sound on devices with host mode capability, and it's very probable that some upcoming minor release of Android 4.0x (ICS) will support it for good.

Poor old N900...