I see two primary options. One is very cheap
and one is very easy.
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.