updated patch with your changes in documentation and pg_dump (initial) support
Main issue of this patch is storage. We can reuse local buffers used for temp tables. But it does allocation by 8KB and it creates temp files for every object. That is too big overhead. Storing just in session memory is too simple - then there should be lot of new code used, when variable will be dropped.
I have ideas how to allow work with mix of scalar and composite types - so it will be next step of this prototype.
