I've had the same challenge - once on a terrace in Melb and again in SYD! Solved it by laying a few of layers of carpet/ underlay/ foam across the bullnose verandah and then laying a LONG wide light-weight plank across it, close to (and parallel to!) the vertical area I wanted to paint. Spreads the load across the rafters supporting the bullnose and the carpet "fills in" the corrugations so they aren't crushed (not a good look). Safety is an issue - try to fasten everything off so it can't move (maybe install a couple of eye bolts and just remove and patch when the jobs done?). Let's know how the job goes!