I can assure you there is massive interest in this feature. Being able to change infill parameters for parts of a model saves a lot of time and material on huge models on a Creality S5. Why infill the whole model at the densest requirement when only a small part needs dense infil? And sadly this part can not be separated by layer height.
I'd like to vote for this as well. Slic3r isn't very good in other regards, so S3D is still my slicer of choice. I can't really do threaded inserts with S3D, as I need to change the number of perimeters for the entire part rather than just the places the inserts go.