I've only ever been to one big con (DragonCon 2014), but I'm pretty sure not only would you lose signal very quickly, but you would also pick up on everyone else that had the same idea, assuming you're using commercial grade "cheap" walkies.
But as for uses of the HUD, the last latest and greatest thing I've seen when it comes to HUDs, are the Oakley Airwaves. They are advertised to connect to your phone via bluetooth, and work with a downloadable app to send you placement, friend placement, direction, text message/notifications, and a couple of other things. Might as well link it:
http://www.oakley.com/en/airwave/product/W0OO7049There are others out there, mostly for sporty things like biking and such, (
http://www.reconinstruments.com/), but for me, I think I would use it to bring the fandom out even more. Also, I'm a huge proponent of technological advances, and I see this a huge one, or in the words of another man, one giant leap for mankind. Honestly, if a group of nerds like us built a working, customizable HUD and shared it with everyone, it might cease being a nerdy thing, and see widespread use. Bring on the future!