NA and blown classes currently exists in the rulebook as Rich mentioned. If no record is listed that indicates that the class is open and the fruit hangs low on the branch.
If you look at the body class you are interested in at the bottom of the description there is a list of allowed engine classes. Section 1.F states that if no record is listed then it is an open record at Bonneville(zero MPH). Two consecutive runs will get you the record.
El Mirage is slightly different in that there are published minimums for open records. These minimums can be found in the current revision of the El Mirage Procedures manual found on the SCTA website,
www.scta-bni.org. Look for El Mirage on the left side, click on Procedure manual and scroll to the bottom.
DW