I think the term you are looking for Jimmy is 'wildcard' entrys.
I like the idea. 5 reserved places for wildcard drivers, if no wildcards want in the reserve drivers are called up a week or so in advance.
the 'limit' should be set at 125 (for 2WD) for entry to the event with 5 reserved for wildcard or reserve (as long as wildcard entrys were closed and reserves given plenty of notice)
this happens quite a lot in other forms of motorsport, things like superbikes, supermoto etc... 'local' hero's get entry to the 'world' event at their track etc... they normally do quite well
