Offline page as last resort with GTM

So why don't you use Fallback IP? The IP will be one of the virtuals from your main pool. On that LTM virtual will be an iRule that has an LB_FAILED event showing a sorry page. It will have a pool attached and so will show as up/down on LTM status.

Seems way simpler than mucking about with Last Resort Pools on GTM and LTM virtuals with no default pool attached, and definitely fulfils the "can you get the GTM to use an LTM VS even if it is flagged down by iquery" issue unless there is more to it than that?