Wouldn't it be amazing if all these were multi-valued properties? That would really open things up for companies with many filters necessary to accommodate various things (e.g., DDG's, ABP's, Transport Rules, etc.).
I imagine that converting in place would be difficult if not impossible. However, adding one new property - CustomAttribute16 - and making it multi-valued could be easily carried out in an Exchange CU that extends the schema.
Will just leave it at that, and hope somebody loves it enough to make it happen. Thanks in advance.