Load balancing / proxying AD itself?

Does the iApp also allow you to specify the domain name? Active Directory is pretty good at load balancing itself (and is recommended). If you can specify an AD domain instead of a DC server address, the system should perform a SRV DNS lookup to find the active/preferred DC.

Otherwise, if you know the ports you'll be using to talk to the AD (ex. 389, 636, etc.), you could create a VIP on that port, create a pool of AD servers and use an appropriate monitor, and point your iApp at that VIP. That said, you're relying on the (external) monitor to tell you the health of a domain controller - something that is usually better left the AD itself.