I think I did. It shouldn't be any meaner than "output_signature->element_count + some extra stuff" though; it's a different bound, but it doesn't say anything about the order by itself. I wouldn't necessarily be opposed to specifying this as e.g. a sorted array of "{src_index, dst_index, dst_mask}" either though.
v4 does this, albeit without the sorting.
I also went ahead and added a patch to avoid outputting semantics not read by the next shader stage; it ended up being simpler than I was afraid it would be.