I have had an SUV and a sports car, both accumulated brake dust (especially the sports car if you drive the
right way. A.K.A. the way they
should be driven.)
On my Porsche 911 and my dad's Aston Martin Vanquish, I have sealed the wheels with Opti-Coat 2.0. Now, and here is the hard pill to swallow, it does
not prevent the brake dust from accumulating. Sorry if advertised differently. On performance cars brake dust happens (watch any race, Formula 1, IndyCar, ALMS, Grand Am, etc. and when they change tires (or should I say tyres for your F1 types

) and you will see loads of brake dust come off, and these guys are using ceramic rotors and pads).
Where the Opti-Coat 2.0, or any other sealant, permanent or otherwise, comes in handy is during the maintenance cleanings. It just makes them easier to clean and prevents the dust from etching into the wheel. I Opti-Coated just for the insurance policy of not having brake dust scratches, not necessarily to make cleaning any easier.
Personally, I think that rather than worry so much about keeping brake dust off of your customer's wheels, I would try to explain to them why it is accumulating and manage their expectations a little more. If they brake, especially hard, it will accumulate. If they don't drive, it won't.