What is the best way to copy data between arbitrary tables? Or once again about 'null':jsonb

[thedolphin]

I'm writing "yet another specialized smart data migrator" for project needs, and got stuck at jsonb.
Table has jsonb not null field. Upon reading 'null' value decodes to nil, which then can't be written back as it encodes to NULL.

Is there some generalized method to pass data from conn.Query to conn.Exec and conn.CopyFrom which correctly passes field types?
Another way is to use rows.RawValues(), but how can I use it in conn.Exec and conn.CopyFrom then?

[thedolphin]

Use underlying pgconn directly, Luke!