As Matt suggested, the inband monitor only applies at layer 4. It wouldn't necessarily make sense to try to check a layer 7 application response code for this. You could argue for a layer 7 inband monitor which marks the pool member down on specific response codes.
Though this is generally too strict for most customer's applications as you wouldn't want to consider a server down if it generates a single HTTP 500 response. Marking a member down on a specific response code in a potential new monitor type or in an iRule would probably open yourself up to a denial of service scenario if a single attacker can trigger an HTTP 500 from each server in the pool by crafting a bad request.
Aaron