As I understand these schemas (which has been very little up to now) there is "supposed" to be a standard format/schema and then a section of possible custom data. The trick in this case is to read the standard data the same way in every case but then pass the rest of the data according to another set of rules, say pass thru. However, in practice, as I've heard several Mainframe folks claim that the standards are rarely followed and new ones are created depending on which way the wind blows.
That said. I'll keep this issue in mind and review some internal work to see if I can find anything. I'm interested in a solution as well.