This may or may not work. By placing a siding panel on top of the insulation you’re compressing it and lowering insulation’s thermal value. Decompressed insulation has air packets between the fibers which work as an insulator.
Second problem is that by placing a siding panel on top of the insulation you’re adding a second moisture barrier / vapor retarder. Whatever the amount of warm air that was penetrating the ceiling, R-19 insulation layer, and condensing on the roof’s decking will be now condensing on the siding panels surface, soaking the insulation, and dripping onto the ceiling’s drywall.
One of the attic or cathedral ceiling insulation
rules is to have only a single vapor retarder installed on the side of insulation facing the house / conditioned space. Never trap moisture absorbing insulation between two vapor retarders.
You can try your idea it if it doesn’t involve much labor and investment. In case it doesn’t perform well you can always take down the drywall, staple vent chutes between the rafters, and re-install insulation. A lot of work but I’m not aware of any other quick solutions to your problem…