Are we stuck with Schemas?

The schemas for tables seem to be very limited.
Something so simple as creating a bonus against a single unit <type> is not present anywhere.

All I am trying to do is give a unique unit a bonus against barbarians. I can't find any way to do this in the schemas for units or promotions.

The only reference to barbarian bonus I can find is in the schema for policies, but it doesn't seem like I can apply it to anything but a policy.

Is there a way to add to the schemas? Or to move elements from one table to another?
Is there any way to create custom promotions like this at all?

You can add to Schema's very easily: http://forums.civfanatics.com/showthread.php?t=393085

Yea, it certainly seems that we have full access to the SQL DB (I've only run a few basic queries so far so there may be some limitations, not really sure), so you can do whatever you want to it. I assume you do realize though that adding new columns to the DB won't by itself do anything at all, and you'll need to write some Lua to actually read the data out of those tables and do something with it.