Which type of Resource are you referring to - JBossAS Server, Tomcat Server, or Apache Server? Also, what are the names of the metric(s) you are referring to?

I just peeked at the Apache plugin's descriptor (META-INF/rhq-plugin.xml inside the apache plugin jar) and as you said it defines a metric for 500 responses, but not for other 50x responses. The 500 responses metric is defined as follows:

<metric displayName="Number of 500 Responses" property="wwwResponseOutResponses.500"
                 description="The number of 500 responses generated by this service"
                 defaultOn="true" defaultInterval="300000"
                 units="none" measurementType="trendsup"/>

I believe the SNMP Apache module that exposes the various metrics does support all of the other response codes, so you should be able to add additional metrics for 501 etc. by just adding some new metrics to the plugin descriptor, e.g.:

<metric displayName="Number of 501 Responses" property="wwwResponseOutResponses.501"
                 description="The number of 501 responses generated by this service"
                 defaultOn="true" defaultInterval="300000"
                 units="none" measurementType="trendsup"/>

Note, I had to change "500" to "501" in three places.

After adding the new metrics, you'll need to deploy the updated apache plugin to the RHQ Server and then to each of your Agents. To do so, follow the instructions here:

http://www.rhq-project.org/display/JOPR2/Adding+and+Updating+Agent+Plugins

If you'd like to see the new 50x metrics added to RHQ, create an enhancement request in RHQ Bugzilla and optionally submit a patch.