I have a USB thumb drive containing a live boot environment.
It's actually set up like a HDD with a single ext2 partition within and boots a minimal customized version of linux.
How would I go about setting this up to boot via PXE (or iPXE)
If you've got a functioning PXE server, you should have no problem using memdisk with an image of the drive. I've used memdisk to boot a hybrid ISO over PXE, serving the ISO over TFTP rather than NFS.