> Well, since the extra code will be autogenerated, guess it wouldn't be much problematic. 

It will because it means that generic functions that receive asdl_seq an need to cast aliased pointers because they will still receive asdl_seq* items and you now need to cars them to the appropriate type:

int foo(asdl_seq* my_seq) {
    asdl_seq_expr* = (asdl_seq_expr*)my_seq
}

So you will end up with what I am proposing: a flag in asdl_seq that tells you of what type it is.