Fallback Host Conditional Trigger

I think you'll have to do an iRule

The 'easiest' (But perhaps not most efficient) would be to

1. Make a sideband connection to your URI you want ot check
2. If the sideband worked, then just return - no action to take
3. if the sideband didn't work, then send a redirect to your sorry page (Or if the sorry is on a pool select that pool or whatever)

Now that's not going to be very efficient. That sideband connection will get awfully busy. But it's easy.

A more efficient method would be to replace that wideband query with an explicit pool. And the monitor for the pool checks /probe. And do the redirect to sorry if the pool itself is offline.

You can then add more bells and whistles as you require. But try to keep it simple. Simple things fail far less.