P laying with a lot of things tonight, but I'm still far too disorganized to be productive. I ended up just dabbling with NodeBeat by Scitta. It's one of a class of apps that make fuller use of an iPad's touch interface to manipulate things that relate to the sounds being produced. In this case, you drag nodes around that connect to beat generators, which trigger as soon as a signal connecting them arrives. The way you set up the geometry determines how the music comes out.

NodeBeat is a collaboration between Justin Windle and Seth Sandler, and one done a while ago at that (2011, though still maintained), and it doesn't look like they've done more with this, though they each have been up to some other neat things.

When composing (positioning the nodes around, adding and subtracting), it's impossible to predict exactly how things will go, so you play with it live, and adjust as your whims change. This is how it sounds with the settings I'm using:

I'm not saying it's something I'd listen to, but it's certainly interesting, and I could imagine using it for something more mature. I'm not sure what the limits of this particular app are yet, but I like this kind of composing interface.