Hi Ross,
You can use LB::status (
Click here) to check the status of a pool member or node. Per the wiki page, this command was added in 9.1.2 HF4. You can download 9.1.3 (which I think has the command) or HF8 from downloads.f5.com.
Do you have multiple rules which contain the global variable, ::HOSTNAME1? It's possible that one rule is conflicting with the other. You can add a log statement for debug purposes to check the value of the list in the RULE_INIT event and the HTTP request event:
log local0. "class value: $::HOSTNAME1"
The output should be something like:
rule : class value: {host1} {host2} {host3}
Aaron