For utilitarian purposes, a decent rack trunk secured to a good rack is an excellent solution. But since OP explicitly stated that he's looking for a classy way to attach a saddle bag to a Brooks B72 (which is, if done properly, also an excellent utilitarian solution) I don't think that's what he's looking for.
A rear rack does not serve well as a saddle bag support. I've tried it.
I respect that the OP has considered what he wants.
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But I have to wonder why? The big saddle bag seems inherently floppy, high up, possibly awkward. I just wonder whats the attraction.
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