iRule redirection for VIP on 443

using f5 dns gslb is ideal solution and it is done is dns layer.

this is not exactly your use case,
but you can use this article as example how to make http response when all pool members are down.
https://my.f5.com/manage/s/article/K20405069

in your case, use http response 302 (temporary redirect).
dont use 301 as it is permanent redirect.