Money plus charity contribution before the event, with collection of bands/id's when at LM, seems the most sensible, keeps the amount of cash on site down, postage costs at zero, and cannot be mistaken by the ACO as a commercial exercise.
Fairer than sponsering a barrel, for the smaller groups of people.
However you id the contributers, I would not let them know how, till they collect it at LM, unless it's very unique, otherwise the freeloaders will come prepared.
UV idea good, as are the unique mugs, which would solve the problem of someone going to the bar to get drinks for a few people.
I'm sure there is enough CA's (I'm one) who would be willing to take a turn behind the bar to give the DfH guys and girls a break, there are so many of us, a 1/2 hour stint, (if there are enough volunteers) would probably be sufficient, although you might want DfH there towards the end on the evening