Not exactly what you were looking for, but you can connect from PostgreSQL to MongoDB via Foreign Data Wrappers - https://github.com/citusdata/mongo_fdw
I know about Foreign Data Wrappers and use them extensively. To clarify, what I want is to have a quick engine swap for some legacy apps without touching anything but config files.
Haha, did you just refer to Mongo in the context of legacy apps? I feel old and tired, and we're not even 10 years into this....
Well, I guess "legacy" doesn't always mean "old" - not that I didn't have a weird smile while typing that particular word.