This isn't really that different to GWT, which Google has been scaling for a long time. My knowledge is a little outdated, however more complex applications had a "UI" server component which talked to multiple "API" backend components, doing internal load balancing between them.

Architecturally I don't think it makes sense to support this in a load balancer, you instead want to pass back a "cost" or outright decisions to your load balancing layer.

Also note the "batch-pipelining" example is just a node.js client; this already supports not just browsers as clients, so you could always add another layer of abstraction (the "fundamental theorem of software engineering").