Take it or leave it as far as credibility goes but I've used rear tyres running the opposite way on the front. It's not uncommon in sidecar circles because it gives you a flatter profile.
I used a 130/17 mitas e08 as a front rather than a 120/17. Don't know if that helps with tyre choices?
You could also look at the Metzeler Enduro 3 Sahara. This has the tractor tyre like profile of the mitas e07 (what were OEM fit on the ktm690, don't know if they still are).
I would use an e09 rather than tkc80 personally. Have you thought about a wheel swap to 19" from the other Strom's. Don't know what's involved but that would give you a good choice of front tyres. Then you could just drop the forks through the yokes a little to offset the 1" lift from the larger wheel.
Personally I find two sets of wheels the next best thing to two bikes.