Run command before backned is marked as UP in HAProxy

I have to solve this problem. I have two memcache servers (SM1 and SM2) and HAProxy loadbalancer in front of them. HAProxy is configured in active/passive mode, when SM2 has backup command. All traffic is routed to only one server at any time, switching only when active server fails.

I need to ensure memcache is clean before server goes UP. Our application cannot detect if data in memcache is too old and will use it (it can lead to some problems). How can I send flush_all to server before is marked as UP? Is it even possible? Or is better to switch all application to redis (which we're using to store sessions data) with replication? Thanks a lot.