Koala Sampler - Excellent (mostly) for Audio Tracks!
Well ... not as a replacement for them, but as the best workaround for now.
Koala Sampler just gained AUv3 capability with a low-priced In-App Purchase. It works fabulously as an audio clip launcher for NS2. Much better than Multitrack Recorder IMO.
Koala has 64 pads that can be loaded with audio samples. Each can be triggered by midi note. It has very good realtime audio stretch for adapting to tempo changes. Audio can be set to one-shot, looping, with hold or not. What this means is you can load up your samples, then add patterns to the timeline with a note to trigger patterns. I prefer to make one pattern the length of each for each clip so that I can give them names and have a visual indication of the length.
I far prefer this workaround over Multitrack Recorder for the excellent time-stretch, ability to assign clips to choke groups, and better visual representation in the timeline. There are some fun FX to play around with too, and they assign nicely to the macro knobs and XY by default As with Multitrack, unfortunately you can't record external audio inside NS2, but it isn't too hard to record in the standalone app then import to the AUv3.
I feel like this makes NS2 far more suitable to my needs while waiting for if/when audio tracks arrive.