This may be what you mean, but a common approach seems to be to create extra tables to create sort-of-implicit-schema-less-indexes, e.g.,:

http://news.ycombinator.com/item?id=496946

A PostgreSQL-specific alternative might be to write triggers in one of the provided procedural languages to turn your JSON into something indexed or materialized elsewhere.

Do either of those work for you?

Also, purely out of curiosity, do you have a design reason for only wanting to store schema-less JSON, or have you just been burned by slow database migrations in the past?

There seems to be a big community of people who really want to reject schema and use JSON for everything, and I'm really curious if they (a) don't understand relational databases, (b) are getting some surprising productivity gains somehow, (c) have been burned by slow database migrations in the past, or (d) some other reason.