If I'm not mistaken this is caused by the fact the EqualLogic provides block systems and doesn't know what's actually in use on your filesystem so it doesn't have a OID for the filesystem usage. We had the same wish for our VMware volumes for which I ended up monitoring their statistics trough vCenter because it does know how much is actually in use.
We use a third party app to monitor disk space usage and been looking at getting our 6 volumes monitored using SNMP. I have managed to identify the Volume OID's (1.3.6.1.4.1.12740.5.1.7.1.1.8.), which gives us the total size by using eqliscsiModule.mib But cannot seem to identify the correct OID to read the used/free Volume disk space. Can someone help?  Thanks.
PS6510 and Volume OID's