I did the same trick with GIMP_DEPRECATED_REPLACE_NEW_API macro, apart
for some minor widget API to preview drawable, which I will fix right
away in our plug-ins.
Validation is not the job of a GValue setter, we have the GParamSpec's
value_validate for that. Also, we will use the new GType-based API in
the libgimp PDB wrappers soon, and it's really a bad idea to
implicitly call e.g. gimp_image_is_valid() from the generated
gimp_image_is_valid() wrapper.