I was trying to do a similar thing a month ago. Playing Samplr over USB using studiomux in Ableton works great if you are using samples already stored on your ipad but recording new samples over USB turned out to be not as straightforward as I hoped.
Studiomux wants apps to be instruments or effects and if the app is an instrument it doesn’t seem to receive sound in. Some apps show up in studiomux as both instruments and effects but Samplr only shows up as an instrument. The solution I found was to run Audiobus inside Studiomux and run Samplr inside Audiobus. However this doesn’t seem to work with Studiomux’s VST connection so I had to just make an aggregate device for audio in and out and send audio over the USB using ableton’s external outs. And then I couldn’t figure out how to sample into Samplr without having the audio also come through over the ipad speakers.
I was working on this for a performance and this solution crashed too often so I abandoned it.
The other solution I was going to try was to send audio from ableton out my interface and into the audio jack on the ipad, but I think that requires a TRRS adapter which Rode makes and I just haven’t gotten around to getting one yet.
You can use Audioshare to transfer samples recorded in Ableton to the ipad and Audioshare can send the samples to Samplr. Something I haven’t tried yet is to see if there is a way to record to from Ableton over USB into Audioshare instead of using wifi to transfer samples.
What you describe does work with other apps. I’ve been sampling from ableton into Borderlands and playing it back into ableton with very few problems.