(NB: Post author here)

Yes! You can use any of the hyperfunction aggregates to build a continuous agg. Including sketches like the percentile approximation stuff.

With hyperloglog, it's still experimental, so you have to jump through some hoops to get it into a continuous agg as it could be dropped on next release, but once that's been stabilized it'll be very easy to use in continuous aggs. Feel free to test now, just know that it may get dropped and you'd have to re-create the continuous agg after the next release when it gets stabilized.