I believe that the mobile phone system has progressed beyond that now - at-least in the UK, although I'm less sure of Eire.
I'm not an expert on the height of fireworks, but that sounds a little low to me - has somebody done a formal safety assessment on this?
Flight test departments use a telemetry system for such things; you could try speaking to somebody like BAE to see if they have an off-the-shelf system that they can make available. For that matter, what about the BBC?
I do know of a small company called "Airborne Engineering" who do such things (they were closely involved with the edge of space balloon project). No idea of specific capability or availability, but you could do worse than contact a chap called James Macfarlane, Email james (at) airborne-engineering (dot) com (do the usual reconstruction; he won't thank me for putting his Email address where a web-bot will use it to bombard him with spam).
G