Load balancing NTP Servers

First question would be why?

ntp is perfectly capable of doing its own load balancing. And results in a much better result. For instance your big probably won't know whether the A NTP server is even any good (It might be responding with the wrong time). The ntp protocol includes logic to avoid this (i.e. if the time is too far out). 
As long as you have multiple time servers (3 at a minimum, but you really need 1 more to provide quorum in a server failure scenario) ntp will work it out way better than checking a big in the way.

Also why would you monitor an ntp server via https? Just because the https responds doesn't mean ntp is going to. You monitor services via the same protocol as being served. That way you have a better chance of knowing when the service is up or down.