+1 for this feature
I have a similar use case for such a feature.
In MS Expression Design 4 you have a feature called "segments" (I know this word is used otherwise in the area of vector graphics). These segments store their area together with the currently activated layers. You can freely define segments and everything visible through the area of this segment will be exported to a predifined image format. In the export dialogue its possible to simply click "Export all segments" and every segment you have defined in the document will be exported with one click. This is useful when you generate several views of one and the same design. And when you change something, you can export all parts with one click.
I had a short time trying this out in my test period of the Affinity Designer, but as far as I saw this was not possible with slices. I don't always have parts organized in a grid like manner. The partial views I'm talking about can overlap and have specific layers visible and others hidden.
I also described it in the following forum post: