I see two primary options. One is very
cheap and one is very easy.
The
cheap option is to configure your 802 to transmit on the ham bands and pair it with a PC
laptop to use Winlink. We bought a
laptop exclusively for this purpose ($180) but if you already have one then the costs are minimal. However you'd need to pass your technician and general amateur
radio licenses. If you have experience with
electronics or are very familiar with
radio you could probably pass the tests outright, otherwise you can study a bit.
The easy option is to buy an
Iridium Go! ($700) and one of their basic plans ($60 a month). It's just
plug and play and cheaper than getting a
Pactor modem for your 802.
EDIT: You might be able to rent an
iridium Go! for a couple hundred bucks to use for the event. However if you're planning to participate in more rallies in the future a permanent email solution might make more sense.