Did you make sure to switch the USB input on the tablet to "MIDI"? You can do this on some devices by dragging down from the top of the screen, tap the MIDI entry in the status and switch it to MIDI. You can also get to this within the tablet Settings app. This will ensure the tablet will show up with MIDI input and output ports that other applications can use (I've utilized this to connect it to things like loopMIDI).
As far as Ableton Link, it's open source, but the license is one where proprietary/commercial applications can't use it without first contacting link-devs@ableton.com. I'm guessing they may charge some kind of licensing fee. I don't have time to look into this right now, but I can revisit it in the future if enough users request it. The LinkKit for iOS doesn't seem to have the same licensing restrictions.
Mike
As far as Ableton Link, it's open source, but the license is one where proprietary/commercial applications can't use it without first contacting link-devs@ableton.com. I'm guessing they may charge some kind of licensing fee. I don't have time to look into this right now, but I can revisit it in the future if enough users request it. The LinkKit for iOS doesn't seem to have the same licensing restrictions.
Mike