S3_PING still seems to be an experimental feature and that might explain the missing documentation... Anyway, you can configure S3_PING to the jgroups-channelfactory-stacks.xml that is situated in
See JGroups Channel Protocol Stack Configuration in JBoss 5.1 Clustering Guide for more info on the contents of that file. Sadly, as mentioned before, it doesn't say anything about S3_PING. Basically, you can just replace the PING element with something like this:
<S3_PING secret_access_key="903rfj04rjg490" access_key="ji3gfo" location="bucketname/>
You might need to create the S3 bucket by yourself beforehand.
It seems that JBoss 5.1 doesn't work with the newer JGroups versions that have support for S3_PING. By looking at JGroups JIRA, it seems that there won't be any support for newer JGroups versions in JBoss 5.x ever. This means that you have to wait for the JBoss AS 6.0.0 MS3 version to get it, see the issue here. By looking at JBoss AS JIRA, it seems that MS3 won't be released in the near future. :(
I just noticed that the guys in the JBoss StormGrind project have made S3_PING work and asked more what versions they use. The use JBoss AS 6.0.0-MS2 and JGroups 2.6.13.GA. So, S3_PING was released before 2.8, even though some pages in JBoss site claim otherwise. I haven't yet tested this myself.