OK, got it – I just wanted to talk it through so that I wasn’t trying to answer a different question due to not quite getting the goal.
If you absolutely require switching between a grammar and an ARPA model the only recommendation I can make is to use RuleORama, since it outputs a DMP which means you can switch between it and your normal ARPA model that you want to use n-best with. Unfortunately it is neither possible to support switching between JSGF and ARPA inline nor to support n-best with JSGF so I don’t see a way to do this precise spec with stock OpenEars.