Allow unorthorised access to a single REST service

Hi,

I've added a REST service which worked fine on our test system but failed on the production environment because UnknownUser does not have %All set and I really don't want it set on production (in fact I've also switched it off on test).

Is there a way to allow a single REST service to have unauthorised access?

I was thinking adding a resource/role to UnknownUser specifically for that service but I've never touched on Users/Roles/Resources so I'm struggling to work out what needs adding where.

Thanks