Thats been the norm in ECEA for years to keep the man happy. Public roads are used as connectors, plus state land is used in most events. In reality, it is not enforced on the course, just at signup. The state cops and most of the rangers are cool as long as your not doing something real stupid. A few years back, nothing was checked and you could slap the plate borrowed from your street bike to your woods bike and go for it.
Since land use is becoming such a problem in recent years, primarily from fear of litigation due to the over breeding of lawyers, the latest trend is toward closed course events where possible. If held entirely in a state park, a "parade permit" is obtained by the club, and the roads in the park are closed. In this case a pure MX bike (with a spark arrestor) is acceptable.
roost, any plate (and registration/insurance) will do. GGs can't be registered in NJ so I have a VT plate.