Not sure about the Vaya, but the Gryphon has an EBB – no tensioner required. Singular has large and extra large in stock, by the looks of it.
FWIW, I have a Specialized Tricross singlespeed frame built up with a Nexus 8 hub – the track ends are 120mm, and you can reduce the width of the Nexus by removing some of the coaster brake fittings on the non drive side. No discs, unfortunately, but I have to say I’ve not massively missed them. Would be nice, but not vital for what I’m using it for. You’re probably using it for something else, mind.
One question: why not get a custom frame made? Might be worth pricing something with Black Cat swinging dropouts up. A little extra could get you exactly what you want, and it’d be a keeper…