Its an Unregistered Vehicle Permit, and is only valid for a MA sanctioned event. The give no fark for compliance plates or most of the rest of the full-rego guff, because they recognise that the bike will be doing very little on-road work, the bike will be scrutineered, and you're not using it for commuting to work/getting a loaf of bread from the shops.
Its the same scheme that they use for moving tractors/earth moving equipment etc.
Dunno if you need a RWC for a UVP on a bike? Rally cars don't. If not, then you've got to fill out a declaration of road-worthyness, which asks questions like "Does it have brakes?" (no kidding - tractors and stuff, remember).