as the cannister gets dirty the flow is reduced. for some reason I was think h.o.b filter then plumbing it to the sump, but rereading your post I guess the rim is to thick for that.
That's my only problem I can see with this idea. I have like a spill tray that's about 3inchs high that all my equipment sits in so if the sump was to overflow it would run into this tray. If I have the intake pipe close to the top then surely the siphon would eventaully stop when the water level drops and it draws air in? I know it would have probably overflowed by then but my tray would have caught the water so then I would only need to remove that water and clean my tray.
You could drill a hole on the intake pipe (inside the aquarium) somewhere below the water line and see if the gravity siphon breaks once the hole is above the waterline.
this could be customized to fit your bracing