USB OTG support for lumia series...

True, I didn't think of that. And I guess one would need an USB-hub with external power to connect multiple devices?

 That's what I thought. I haven't bought the DAC yet though. My choice is between Fiio E11k (line-out) and E10k (USB-out). The latter would be preferable sound-wise because it would exclude the internal sound chip. The E11k works on all devices though, and is battery-powered. My Lumia 1020 won't support the OTG function anyhow though, if I read the blog post correctly, so I guess the E11k is the way to go.

A DAC is normally a USB-Audio device, which is covered both as source and sink according that MSDN blog entry. Final testing with your particular USB device is required – but theoretically it is supported. Update (December 2015): My Sennheiser 3D G4ME1 works with my Lumia 950. However, I had to insert a power supply to the USB adapter. Furthermore, I have not tested a DAC which follows USB-Audio Class 2.0 (UAC2), yet. That Sennheiser uses the original version of the USB-Audio Class specification.

Don't forget Win 10 for phones suports BT HID (Mouse and Keyboard) so that feature has a solution now. Works well BTW, (L920 tech Preview). Actually found that more convenient back in the PPC/Win Mobile days, than wired stuff that would need a hub. I think your DAC remains to be seen. Certainly possible, but I would guess more than just the OTG needs to happen. The audio system needs to be set up to deliver the data to the USB host port. Hope someone does that for you. For me, although I have more cloud storage than I know what to do with (O365 unlimited) I think the mass storage is what I would use the most.

You're welcome. The suggestion to add OTG supporthas about 22.000 upvotes on the WP suggestion forum, I only found the confirmation from Andrea that they will include it at the very bottom of the first page of comments. One more comment and it will jump to the next page. Seems like they are not doing much of an effort to get the information out there.

Btw, I saw your comment from last year about what people will actually use this feature for. USB mass storage is perhaps not very needed, but other features (like USB-hub with mouse and keyboard) are useful. Personally I was wondering whether I could connect an extarnal USB DAC (like the Fiio E10k) to my Lumia or not.

Good find. Thanks for posting the link. I'm glad they are supporting it, as it is one of those things that others have that just detracted from the platform being absent. Hopefully the OEMs, one of which is MS, will include the hardware  required.