yeah - DVD manufacturing and authoring isn't as expensive as you might think. Granted the filming might be, but a couple of Sony DP170 cameras (as used by the BBC for some broadcast material) hired for the day for the live stuff, a decent editing package like Vegas/Premiere and you're away to a good start.
I agree about negotiating for the bigger bands to get songs might be difficult/costly, but certainly the smaller/unsigned bands could contribute/donate a live song (take the sound from the desk) and you're away.
The other point is that a lot of bands (MR included) are making promo videos nowadays, and a bit like the 'Bloodstock on the Road' franchise, where it's tours for bands who have played Bloodstock, I'm sure that a DVD along the same lines would go down well. Bands involved with Bloodstock past and present could either donate a live song or a promo vid. Great exposure for us lot, plus a great souvenir for people who go to the festival.
FourWayKill
Biomechanical
Illuminatus
Mercury Rain (ahem...)
All have videos out or coming out soon, and I'm sure that other bands have footage.
Doesn't have to to be Dolby DTS and all that stuff, but more 'quick and dirty' would surely be acceptable to most people... if priced accordingly... £12.99 maybe ????