I volunteered at the bus depot one year ... it runs Tues - Friday I think, and yes, you do prepay for the round trip. You had to be sober, dressed, and have your ticket stub. We issued wristbands to get folks on the bus, you had to select which time slot you were going into town; seats often sold out. You couldn't buy a ticket and get on whichever bus you wanted that day. The gate crew collected the wristbands of people coming back in. Pretty much no wristband, no ride.
Two concerns: the drivers do not collect the fare, that means a volunteer at the other end to arrange room and fares (no standing on the bus). I personally wouldn't volunteer to sit in Empire, no matter how noble the cause.
Second, the bus only runs limited hours on limited days. If you didn't arrive in Empire during daylight hours, you wouldn't get a ride. And the bus might well be sold out, the five o'clock return was pretty crowded. No way to get to Burning Man on the first Monday, no way to get back on Sunday or Monday at the end of the event either.
oh, and not much room for gear either.
Check some of this stuff out, I can be kinda hazy when it comes to remembering things. And things may change, YMMV.
Some sunshine, some dust, a little paint and a tutu. Yeah.