Diagnosing high write throughput on RDS Postgres
It turns out that the vast majority of my write throughput was coming from the stats collector.  Because of the very high number of relations in my database, the stats data is unusually large.  I was able to diagnose the problem by temporarily clearing the stats:

SELECT pg_stat_reset()


Which led to an immediate, dramatic drop in my write throughput.  To solve the issue going forward, I've redirected my stats directory to a ram disk instead of to the hard drive, using this setting: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_PostgreSQL.html#PostgreSQL.Concepts.General.FeatureSupport.RamDisk

The end result is that CloudWatch is now reporting around ~400 KB/s of throughput instead of 40 MB/s.