The "paid"/"not paid" thing actually makes no difference if you do something wrong or stupid or do not do something you should have, and a disaster occurs as a result. In law you would still be held responsible.
If you do what you have been trained and instructed to do, and still something terrible happens, you are not culpable... paid or otherwise.
However, there would be an unbelieveable amount of paperwork recording who worked and when, tax implications of dealings with people from off-island etc etc.
I know from doing work for organisations in Scotland, the tax people there get in a real twist. They don't seem to know their own rules.
Perhaps a way of having your marshal's credentials signed by the DSM might entitle you to something.. eg, discount on the next trip on IOMSP. The more sessions covered, the more reward.