it's impossible to override theme_uc_vat_cart_review_table

Because you are modifying the registry directly, there's no way anybody can override that function.

Attached is a patch that does mostly the same, except that the override callback doesn't actually returns the output but it rather call its own registered theme function to theme the new table. Doing it like this you can theme the new table if necessary as well.

Patch is against rc3, but it should work with -dev. Changes are straightforward to read anyway.