Nick, HAProxy provides statistics via socket or HTTP interface. You can easily monitor these stats and run scripts. Some cron jobs and regex should suffice. Specific cases like this are usually not something I would imagine belongs in HAProxy core, since it is not directly related to load balancing but more of a specific requirement.
You could simply configure all 10 servers, 5 of which would mostly be in the down state until your script brought them UP. HAProxy will happily balance traffic to all UP servers, and shift it away from DOWN servers. You can also use the stats socket to mark machines as up or down, so that traffic can be gracefully shifted before after VM power up/down. However, this may or may not be necessary depending on the services you are load balancing. https://cbonte.github.io/haproxy-dconv/configuration-1.5.html#9.2
