OK, here goes: Guitar pickups aren't strong enough to drive a passive diode ring modulator. Even if they were, you'd still have to have a carrier oscillator (which would have to be powered), so it's still useless for your purposes. Hey, if it was that easy, we'd all be Zachary Vex!
Since you're just looking for "a bit of craziness", try a Green Ringer - it's a proven project, and a quick Google about should find you more info than you could ever have asked for.
are you sure (you probably know more than me but said id ask anyway...) im certain i saw one that didnt need any power source except to light the led.
i just want a simple enough ring circuit (volume, oscillator) thats easy to make and wire myself.
ok so i need batterys......hmmmn.....is the green ringer or ring modulators in general heavy on batterys?
Cheers