iRules Recipe 4: Static Maintenance Page

Unfortunately, you will need to work around this change. You may do so in a few different ways. One way is to use priority group activation, just as you say. The fallback pool would have a monitor that always marks it up, but would otherwise drop traffic (either an invalid address in your space, or something like an address in the 100::/64 space, which is the IPv6 discard block. Alternatively, when you wish to disable a pool, you could swap to a dummy pool with this same configuration set up, which will have more-or-less the same effect.

If you wish for F5 to add a feature that allows changing the behavior on a disabled pool, I recommend contacting your F5 Field Sales team, and ask them to file the request.