Re: Shielding Paint vs. Tape
You won't eliminate single coil hum just by shielding the cavity. Have you considered wiring the pickup in parallel rather than as a single coil. I have this on my LP and it gives a great single coil tone but without hum (the mid position on a tele is two coils in parallel).
I don't think theres much difference between paint and tape. I've used both and not noticed any difference. The paint is easier to apply in difficult areas (I've used it in a tele for instance) but it is permanent (obviously you could probably find a solvent to get it off- but would probably strip the finish aswell).
I think you'll find the consensus is that it will effect tone. But it depends on the rest of your kit to whether you'll have a noticable loss of tone.
Swamp Ash Gibson Les Paul Studio
Light Ash Fender Telecaster
Ltd. Ed. Fender Hot Rod Deluxe
...and a pedal or 10