Just got my copy of photo for windows, it's all very exciting and non adobe-ish :D!
I work mainly in 3d so I do a lot of multi pass editing where I'll take all of the component passes of a render and rebuild them in 3d to give me the option of re-balancing, tweaking and colour correcting along the way. One thing that comes up quite often is that there's mask output from 3d to blend between different parts of the final image. These masks are often used in more than one place and should be the exact same data at all time, save for one being the inverse of the other. The idea in 3d with some things is to keep things realistic, adding more to one part of the output should automatically take away from something else. Inside of nuke it's really easy to do this as you can have a "clone" of a layer and you can also link settings. Would it be possible in Affinity to have something like a "linked layer" so that we could have one source of data as a layer (our original 3d pass) and then duplicate it but with a link back to the original layer so that any changes made to the original automatically happen in the duplicate? What we'd be able to do is have a master layer that we can paint on or colour correct as we wish and then on the duplicate layer just pop on a smart filter to invert it. Any of the changes we make to the master mask would be reflected in the other layer automatically.