I meant user scripts might be able to populate filter lists for the main extension to do.
You'd have to show me the API to do so. I'm reasonably sure the uBlock developers would have thought about this, or somebody else.
Based on the conversation so far, I'm pretty sure extensions can block ads
Not really. The extensions can give Chrome a list of things to block, but they can't block themselves, and the lists have to be shipped in the extension.
but the concern was that filter lists would have to be packaged with the extension instead of dynamically updated. User scripts might be a way around that, as they'd allow loading arbitrary code
Arbitrary code only helps you if there is an API to call.