AFAIK the default serializer does not guarantee the order to be stable, but I haven't participated in the development of this feature that closely. So maybe I'm wrong.

Anyway, it should be possible to write a simple serializer on your own - a trivial PL/V8 function should suffice, I guess. And then you can define a CAST using that function (but generally adding casts is a bit dangerous, as it may have unexpected consequences).