I'm finally, and slowly, beginning to work with actual music. Here's the state of things as of a few weeks back. Pedal is working (and a bug there just fixed), and I just added a bounced note as a sub-block, but hadn't yet added the function to rename BlockIds, so it's still called "b28" instead of "bounce". And of course this triggered yet another bughunt when I realized that the cached bounce sub-block results weren't being invalidated by editing.
It's still primitive, awkward, and buggy, but this "bounce" thing is the first hint of my plans actually beginning to be realized. The bounced note has a composed tempo and itself is the second note of an arpeggio, which is beginning to show how these ornaments can be composed. That's the real test: do I reach for these tools when I want a certain musical effect. And of course I don't know of any other sequencer that could do this sort of thing.
It turns out that the mechanisms to make things compose nicely, namely track slicing and inversion, make evaluation more complicated and lead to tricky bugs. I wish I knew a more principled way to get the effect I want, but so it goes. The point is not to have a pretty program, but a useful one.