I've thought about that before and was unwilling to chance it. I don't have diskettes (gee, do they still make those) bu I have a wallet with mag stripes I depend on (oh great gods of the ATM, please yield money!) and so my wallet stays the same place I alwasy have it, my left front pocket. (Not the rear, which it easy to lose or have stolen, or makes a hard lump to sit on and get butt nerve damage).
Here's an idea, take a credit card and/or a gift card you no longer care about, and put it in an extra wallet and leave in the tank bag for a day of riding, then try to use the card. That should be a realistic test to see if it would be a problem. But those tank bag magnets are HUGE.