AFAICS there's few interpretation about atttypmod necessary because only few datatypes binary convertible (castfunc=0) do use atttypmod at all. Most special case is varchar<->text, one supporting length, the other not; both need atttypmod=-1. bpchar<->varchar both allow typmod in a similar fashion.Yes, you can check if they're binary compatible from the pg_cast table....
But nearly all of the interesting cases require you to understand the type's interpretation of typmod, and you can't learn that from a table. How many cases are there where blindly looking for a binary-compatible cast in pg_cast will really do you much good? AFAICS you'd have to set atttypmod to -1 if you change atttypid without knowing very specifically what you are changing from and to.
It's already implemented in pgAdmin3 this way.
Regards, Andreas
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