You can enable ping if you have version 22.214.171.124. You have to install an "update" called:
You can download this from the Cisco website if you have a CCO account. Although the patch is supposed to be for 4.2, the documentation says it is for releases 4.0.x.x, 4.1.x.x, and 4.2.x.x, I have only used it on 126.96.36.199. There is also a patch to turn Pinging back off, in case you change your mind. You install this update the same as you would any cumalitive patches for the system. SECURITY NOTE: the device is suseptable to the "ping of death" so be warned. The patch does not allow for pings from specific devices, it'a and all or none.
Was you able to monitor the ACS in the end? I'm just doing the same, but doesn't seem to be able to monitor the services for ACS, did you have any luck?