It is now possible to use iPXE to directly boot from an ISO image held on a SAN target (e.g. AoE, iSCSI, FCoE). This has the same limitations as using memdisk to boot an ISO image, but without the memory overhead or initial download delay of memdisk.
There's no special syntax: just make the ISO image available on the SAN target and then attempt to boot from it (using "sanboot" or with a DHCP root-path).
(on server) vblade 0 0 eth0 /path/to/winpe.iso
(on iPXE) sanboot aoe:e0.0
I have tested that WinPE and the VMware installer are able to start using this method. I'd appreciate any further testing.