Hi,
Nope, classic IOS can't handle more than 4Gb memory as it 32bit by nature. Therefore CISCO-ENHANCED-MEMPOOL-MIB only appears on IOS-XR and probably IOS-XE(?).
I can check against an ASR1k. Observium already sees the difference between cmp (CISCO-MEMORY-POOL-MIB) and cemp (CISCO-ENHANCED-MEMPOOL-MIB) although the MIB file for the latter is a bit too old. The version bundled with Observium doesn't have the HC counters while the version at ftp://ftp.cisco.com/pub/mibs/v2/CISCO-ENHANCED-MEMPOOL-MIB.my does. A comment in includes/polling/mempools/cemp.inc.php already mentions the HC counters.
Would it be acceptable to try polling the HC counters from there, and falling back to the non-HC counters when the HC counters don't exist? Or would you prefer if discovery detects which HC counters exist and then store that in the database for the poller?
Cheers, Sander