It is probably cheaper and safer to use a recorder and an external preamp instead of a USB interface with a tablet or a cell phone.
The only interesting possibility would be recording a better audio in a video footage with a smartphone.
We all know that recording separate audio and video is the best solution, but if you want something on-the-fly you either use :
1) Zoom q8 or Sony HDR mv1 or similar . Audio is decent but video sucks because it's mostly wide angle / fisheye, which is ok for sports and leisure but I don't like for a music performance
2) A smartphone or a camcorder ( if you own one). You end up with a better, up to excellent video but you get a crap sound. If you want a camcorder with line inputs you have to spend a lot.
A USB interface with a smartphone could be the way to go as video quality is getting better, but it doesn't seem to exist the option to have the interface recognized by camera apps ( only sound recording apps as far as I know).
Do you have a solution?
The marching band I work with bought a Q8 and one of the staff members uses it to record competitions. The fisheye is really pretty extreme - I can't stand it. It does let you see the entire field from the 50, but it looks absolutely horrible, even far away from the press box. If you were recording a relatively close-up concert with it, I think everything would look like an early Beastie Boys video. It's an absolutely terrible camera, and the convenience is not worth it.
I use my smartphone with a tripod mount and get far superior quality video, albeit not as wide-angle so I have to either pan during the show or accept that performers moving to the extreme left and right get cut off sometimes.. I record audio separately with outboard mics and recorder, and then replace the phone audio track with the high-quality one in Sony Vegas.
The only thing I can think of that's more convenient without requiring post work is a decent DSLR with audio input which takes the feed right from your mic preamp or line out from your audio recorder, so you have a safety audio track being recorded there as backup in case you brickwall the camera input (which is easy to do).