My local BMW dealer sells Wheel Wax for not too much markup over retail. It seems to work well but I don't know if there is a better product or not out there. Like you I haven't seen anything marketed as a wheel sealer at any of my FLAPS. My wheels currently look better than those on 99% of the cars on the road now and after washing the car I literally take a MF towel and clean water and wipe the brake dust off the wheels. Been about a month since I applied, we will see how durable it is.
For remedial cleaning I have some Mercedes-Benz "metal dust remover" it is an oxalic acid based product, I don't think that you can buy it anywhere but maybe a DIY oxalic acid solution would work when I run out? I would not recommend this for regular cleaning but for old neglected wheels to bring them back to life it is magical. Brake dust just melts off, scrub with a soft brush and wash with dish soap afterwards, et voila nice clean wheels. I don't like to let my wheels get to the point where I have to use this stuff but I keep buying used cars and wheels and, well, you know.