Paul,
Take a look at the below page to see if the device you are looking to discover as a whole can be appropriately discovered and see what sort of components or metrics are certified:
Data Aggregator Certification Portal
As for the ASA Tunnel in specific, you would need to see if it requires something beyond the general MIB2 style interface metrics like ifHighSpeed, ifHCInOctets etc. If MIB2 values are sufficient then you should be ok, if it requires more than you may or may not be successful. Again, take a look at the certification portal above.
Troy