I'm hoping, in the plpgsql function, to unfurl the supplied json into a custom type or at least an array of ints, and I can't work out how to do that.
You will be unable to cast the array itself. You must use json functions to unfurl the json into a result set with each row containing a single value. You can then explicitly cast that value to integer. If the casting doesn't fail you now have column of integers that can be "array_agg(value_as_integer)" to construct an SQL array of type integer[] (or whatever you decide to cast it to).
You can write you own pl/pgsql function to do these operations.
As your example includes a two-dimensional array you probably will need to json_array_elements_text twice, then array_agg twice, to get a similar structure in the eventual SQL array.
