Here is Ingrid's solution. The treadling is easy, but the border isn't plain weave except when you change from one twill block to the next. It will work with a floating selvedge, though and it is a border treatment.
Beryl's solution. Treadle plain weave along with the twill blocks (depressing two treadles at once). Three picks against two is probably not going to be all that easy to remember. If Wendy had 12 treadles, it would be a straight treadling sequence.
I put this problem into Tim's Treadle Reducer, but the solution also used two treadles at once and the treadling sequence was a nightmare.
This is the clean version for 12 treadles.
Dear readers - do you have a better idea? Let us know and we will post your ideas and give you credit for them!