I wanted to follow up and report we altered the index in question and turned off fastupdate and we have not seen the issue I originally reported since. Just for the record I did:
ALTER INDEX my_index SET (fastupdate=off);
SET vacuum_cost_delay=0;
VACUUM VERBOSE my_table_with_that_index;
We looked into the performance impact on the inserts/updates and decided it was better for us to take the tiny hit on them and thus avoid these occasional blockages caused by the pending list cleanup cycle.
Thank you Jeff and Tom for the help! I have a number of todos still from this too, including adjusting our autovacuum settings and looking into improving that blocked/blocking query.
No problem, done, thank you.
Ah, you're right, that won't help.
Thank you for these great suggestions, I will play with them.