As a user of the API, rather than a library developer, the filter mechanism doesn't make me particularly happy as it adds a few lines of code that does not add immediate value to a program. I imagine however that the better libraries will come with a few 'inbuilt', 'precached' common opaque values to default with, so the end result is something that in the common case wouldn't be any harder to use.

I have no comment on the other changes, which mostly make me happy. Especially the /events route sounds fun to tinker with.