iRule for Priority Group Activation... with a twist

Should be pretty easy, just configure the monitor used with the primary (and only the primary) with Manual Resume enabled. That way a failed primary won't come back up until you manually mark it as up. Of course, you have a risk that the primary fails, then the secondary and even thought the primary has recovered you've lost your service until you sort it out manually. It sounds like in this case it's worth that risk but unless you're a 24/7 operation you might be setting yourself up for some out of hours call outs?