We have one opclass for each type combination -- int4 to int2, int4 to int4, int4 to int8, etc. You just need to add the new strategy to all the opclasses.
Can you clarify this solution ? I think another solution would be external casting
If external casting is to be used. If for example the two types in question are smallint and integer. Would a function get_common_type(Oid leftopr, Oid rightopr) be useful ?, that given the two types return the "common" type between the two in this case integer.