I've been an Affinity user for about a year and wanted to chime in here because I've spent way too much time trying to find a work around for this exact request. My need is related to needing the objects to remain vectors for laser cutting.
Say I have a pattern I've created and want to put this design in a hexagon for wooden coasters. I can use the clipping mask to hide whatever is outside the hex when in normal view but when I go to Outline View Mode, it shows everything outside of the hexagon shape. Which is exactly what Lightburn or Inkscape sees too. That means more laser time cutting unnecessary things and also the inability to put things in certain spots because even if I can't see it in normal view mode due to clipping mask, the laser programs will see whatever is hidden.