Here is a new patch which addresses some of the issues you found.
> - vs SQL standard
>
After reviewing this, there is actually no conflict with the SQL
standard, because the CREATE TRANSFORM syntax in the SQL standard uses
an additional transform group clause which I don't use. So there is no
ambiguity. Hence, I left the syntax unchanged.
> - function types
I changed this so that CREATE TRANSFORM requires owning the from-SQL and
to-SQL functions.
> - create or replace causes inconsistency
>
Fixed this by adding cache invalidation in PL/Python.