Yes, there is, but it is buried in the custom column code.
It is an excellent idea to add such a declaration to FieldMetadata, specifically
Code:
VALID_DATA_TYPES = frozenset(['rating', 'text', 'comments', 'datetime',
'int', 'float', 'bool', 'series'])
I will do it now and submit it for Kovid's perusal.
Edit: Code has been pushed and merged into the trunk.