There could possibly exist some code segment in PXElinux that depends on either 16-bit mode, or lagacy Bios calls, but I do not a priori know that with any certainty!
Yes, PXE requires real mode and BIOS interrupt services.
For what it's worth, I first was looking at gPXE on coreboot in order to run PXE, because I had been happily using PXE previously on COTS hardware.
But after realizing that PXE requires a standard real-mode BIOS, I gave up on PXE under coreboot, and moved to simply booting a mkelfimage binary transferred over HTTP. That could be done with etherboot, and I think that generally people are happy with rom-o-matic ELFs as a coreboot payload for that purpose. However, since the gPXE project has newer things that etherboot will never have (like support for more, newer ethernet controllers), I felt it was worthwhile to get gPXE building and booting, at least in a pared-down form, under coreboot.
Chris.