Hi! Thank you!
That will unfortunately require me to use a separate file format for the assets, so I'll need one .afphoto to be able to rely on the full range of Affinity's features, and a separate file for the asset that Unity reads. For a project with hundreds or even thousands of texture assets, way too much time will be lost opening the right .afphoto file then exporting it over the corresponding texture file every time a file needs to be changed (and each tend to be multiple times). Even though it may seem like it should be a quick thing to do, the sheer quantity makes it a big deal (imagine for example if all websites took 10 extra seconds to open).
Right now I can make it work, because I enabled "save over imported PSD files" in the general preferences, but it means I need to memorize which of Affinity's features I cannot use because they're not supported by the psd file format. It's also a shame to miss out on them.
I guess an alternative would be if Affinity can automatically save a .png file with the same name alongside every .afphoto file at all times. It'd not be perfect since there would be two of every file, but it'd definitely be an OK solution!