Since this commit, the couchdb_os_proc_pool tests fail consistenly on Windows. It appears that timeouts are being hit. The reason for this is unclear.
Consulting with rnewson I decided to spin up a dev\run single node instance, give it a sample database with docs, and create more ddocs than couchjs.exe processes (7 spawned on my machine). I then proceeded to modify the ddoc map functions such that the emits would change. I validated that these changes populated in the view results correctly. As a result it seems that round-robin use of the OS processes is working correctly, though it is not an exhaustive test.