I've seen them on a guitar once, loved how it looked, I don't really believe in "things were made to be a certain way". I'm more of a "rules are made to be broken" guy.
...there is also a shim that is “oversized” for use when you are installing a dogear style pickup into a guitar that has been routed for a humbucker sized pickup. It spans the extra width of the humbucker route, and adds an extra distance of 3/32” all the way around. It’s cut out of 1/32” thick material.
AFAIK, you can find what they call "goof hiders" meant for the old Deluxes with minihums. The minis and P90s are the same size, so they'd work on a P90 guitar.
PS- I would like a set for my SG200, similar vintage as the SG Pro, they must have used the same routing tools... the SG200 had single coil pickups with huge pickup cavities, no 'normal' pickup ring will cover them. I currently have humbuckers in this guitar and goof rings to hide this. Here's a pic of this guitar in 'mid' project state, and without the goof rings, http://www.facebook.com/photo.php?pid=6638406&id=711762428&l=b17e2e7890