Bit OT. I have considered this, or at least mixing up a weekend at a festival with some cycling (i.e. the Green Man in the Brecon Beacons with some off-roading). However, what do you do about bike security?
I'm not particularly paranoid about it, but music festivals....Unless they've changed a fair bit in the last 10yrs, I wouldn't even want to take a decent tent.
If you stick to small festivals in remoter places like Hebfest (Stornoway), Orkney, Shetland, security isn't such an issue and you can enjoy great cycling as well.