The only useful reason I can think of for having someone's UDID outside of an app would be for a beta distribution service, in which case you're almost certainly better off using a platform such as Testflight/Apple Beta or Installr (personally, I find Installr far easier to use and provision devices than Apple/Testflight).
If that is what you're trying to do, definitely give Installr a shot - it can connect to your Apple account and automatically provision devices, I've used it to send several betas or private builds to non-technical people and its by far been the easiest set up process.
Otherwise, mikep is right, it's iffy territory and I think Apple would disapprove of it / users are highly unlikely to accept the mobileconfig file anyway
Installr
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