Yes, the parallel aggregate basic patch is almost ready. This patch is based on your earlier combine state patch. I will post it to community with in a week or so.
That's great news!
Also note that there's some bug fixes in the patch I just posted on the other thread for combining aggregate states:
For example: values[Anum_pg_aggregate_aggcombinefn - 1] = ObjectIdGetDatum(combinefn);
was missing from AggregateCreate().
It might be worth diffing to the updated patch just to pull in anything else that's changed.