I haven't worked with a PAYG SIM, but I know some providers offer a way to view your current balance by sending an SMS message and they'll SMS you your balance back. It's certainly not slick and probably provider-dependent, but it might be slightly more turn-key than actually having to slip the SIM card into a cell phone.

Another way might be if the provider has a way to consult the balance online, either connect to the site from the device or, perhaps, have an Internet server periodically query the balance and send it to the cell phone.

Probably none of these are what you're looking for, but they might be possible "work-arounds." And don't knock work-arounds... some of my friends and I have had a saying since the early 90's: "With Microsoft, the work-around is the solution." :)

Regards,
Craig Steiner