@CantScareMe - theres no reason why the N8 (& other similar nokia fons with USBOTG) wouldnt support external usb dac as long as its self powered (ie does NOT require juice from fon even for the usb chip) & does NOT require alternative usb audio drivers. the other caveat would be the specific USBOTG cable as i dont think a regular data microusb would work. i do believe the USBOTG has all 5 pin connected with pin4 grounded to pin5. i could be wrong but regular data/charging microusb cable leaves pin4 unconnected as its only needed in host mode for ID.
also i havent had the opportunity to test the USBOTG with a portable dac but i cant think of any other issues as long as the above conditions are satisfied. depending on the location of the microusb port, a custom made USBOTG cable with requisite angled connector & appropriate length would also be a good idea if one were to use it as a true porty OTG rig. otherwise it would be exactly as u prescribed so u're golden.