[chrismiller]

Especially because I'd like to invest in a tilt/shift lens and there's afaik nothing on the horizon of mirrorless

 

Surely any manual tilt shif lens would be fine? Af isn't an issue and working stopped down with a mirrorless camera also isn't a problem. There are plenty to choose from, Canon FD, Samyang etc or you could use an adaptor (Kipon) to transform a lens for a larger image circle into a tilt shift lens.

 

Admittedly something like the canon 17mm would be difficult to create.