You need a schema-less approach if you need to create these things at runtime. Relational databases really aren't such a good fit in these cases. You can shoe horn something into a RDBMS with EAV or datablobs, but it won't scale well.
If you're using Postgres 9.2.x or better, there is built in JSON data storage you could use. You can write stored procedures using PLV8, an embedded V8 _javascript_ engine. It's also <flamebait>faster than MongoDB</flamebait> based on what I've read.