At my startup, we have created a geth fork that uses Postgresql instead of leveldb. It took quite some effort...but we are running a full node right now. I think we are going to put in more effort there - we already implemented soft deletes,etc. Smarter people than me can probably build a better data model for the merkle trees leveraging postgres.

It's 2x slower than leveldb (if that matters to you), but given how slow a blockchain really is ...it is plenty fast enough.

https://github.com/RedCarpetUp/go-ethereum/tree/dev?files=1

I believe that it is time for developers to start thinking of single node scalability and infrastructure that is maintainable..rather than convenience.