Is it possible to telnet into an iPXE client? If so, where would I start.
The purpose of this is to remotely control several iPXE sessions across multiple systems.
This is not possible. If you need to remotely control an iPXE session your only option is to enable serial port console and connect the first serial port to some form of serial-to-ip device that allows you to communicate with it over the network.
I would instead try to setup a system that uses an external dynamic ipxe script to decide how the machine should behave on bootup. My
https://github.com/robinsmidsrod/pxe-dispatcher does a very basic thing like this, and it is trivial to replace the <mac-addr>.pxe script for the machine in question and then just remote reboot (using e.g. IPMI) the machine and make it do whatever the script says. I must admit that I prefer this option together with my
https://gist.github.com/2234639 to be able to override what is the default for the given machine. See how I use the menu-default variable and chain off to the main script.