i like magnets and some can be pretty strong for their size, but i'm not sure i understand what you're trying to do.
magnets don't like shocks so while they might be able to hold a lot of weight, i think a few bad roads could cause some trouble. i think the best way to attach something is make some discrete holes and use a couple of screws (with some clever planning you don't have to make holes anywere where they can be seen when the grillblock is of)
thinking about magnets, they would come in handy to test vortex generators. easy to relocate them.
A good source of seriously strong magnets is old computer harddrives. They use those rare earth neodymium magnets to move the heads that move across the hard disk drive.
Anyway some of these magnets are so strong you have to be very careful just handling them because if two of them get close and your fingers are between them its easy to get pinched hard enough to make a blood blister! There lots of fun!