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
BTW now that we've gone through this a little further, it's starting to look like a mistake to me to use the same @> operator for (anyarray, anyelement) than we use for (anyarray, anyarray).