@Christoph-Hart I didn't know there was built-in support for sustains when I wrote that.
UX Sidebar: I expected there not to be one, since the program didn't show me that one existed. So I looked under the "hard-coded scripts" where things like note transposing is, and thought sustain control would be there. Since there A) exists such a thing as hard-coded scripts and curve editors and it didn't include sustain, I naturally assumed it wouldn't do it.
I don't know how well the "if it's not visible, it isn't happening" assumption holds for users of samplers. The UI probably ought to hint that a sustain pedal is built in to the sampler (discoverability.)