Is there a service offering iSCSI/iPXE boot targets over the Internet?
i.e. on a blank machine with an internet connection available to it, it can be configured to look at a public internet host and pull down an OS image as if you'd got a local iPXE server or similar bootstrapping setup.
Edit: It's clear now that block level iSCSI booting is not the right technology for booting over unreliable networks and a local cache defeats the plug-and-go paradigm I was aiming for.
Using iPXE to pull a more intelligent (handles unreliable and/or high latency connections) boot image via HTTP looks like a decent start, but flashing the NIC or chainloading adds a disappointing amount of complexity to the task.
Setting USB booting in the factory and then sending out USB keys with linux images that do the deployment and installation heavy lifting is a much more sane approach to wrangling iSCSI or PXE into doing something they're not designed for.