in case if all of the pool members are down the users should get redirected to the maintenance page, if certain no. of pool members are down users should get redirected to the server busy page.i think active_members could be usable. can you try?
active_members wiki
https://devcentral.f5.com/wiki/iRules.active_members.ashx
Also if there is a possibility to redirect the user request to another virtual server in case if the no. of requests on a currently working virtual server exceeds by certain value.are these helpful?
iRule::ology - Table Based Rate Limiting by Colin
https://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/1086416/iRuleology--Table-Based-Rate-Limiting.aspx
virtual wiki
https://devcentral.f5.com/wiki/iRules.virtual.ashx
Nimbostratus
