Penetration Testing a Load Balancer

Not that I've ever heard of. A good LB is largely invisible from the outside, and in fact, they can even be difficult to detect. There are several tools and even academic papers that talk about how to detect them, such as this one. Identifying Load-Balancers in Penetration Testing

However, beyond merely identifying them, I'm not sure that anyone has ever put together a recommended practices guide for how to pen test one.

I can say this, on behalf of F5: we regularly pen test not BIG-IP ourselves and also have third parties do it for us. Also we have thousands of customers who pen test their own systems (including their BIG-IPs) and sometimes they will share their results with us. The results aren't anything we could share though, except maybe a cover page to show that the test was run.