There's another important use case which you mentioned Greg, that also requires special exemption: compact commitments via mid-state compression.
The use case is an OP_RETURN output sorted last, whose last N bytes are a commitment of some kind. A proof of the commitment can then use mid state compression to elide the beginning of the transaction.
How do you make a special exemption for this category of outputs? I can't think of a very clean way of doing so that doesn't require an ugly advertising of sort-order exemptions.
The fact that we have two different existing use cases which conflict with soft-fork enforcement, I'm quiet concerned that there are either other things we aren't thinking of or haven't invented yet which would be affected.