At my outfit we call it sandbagging. If you're near to a sim and on standby then you can get called in. I bumped into someone the other day and he told me his mate was called in twice last month to sandbag.
Each training event you do raises a training report. However I don't think a training report is raised if you are the sandbag but I do know that if you mess up then you will be treated in the same way as if you were on your LPC OPC LOE. It is however not voluntary. You are required to do it exactly the same as any other duty.
I can imagine this is more common for the LBA based guys with the sim being just down the road? At our bases with the sims it is much more common.