Is it possible to implement iPXE on an EC2 instance so that it boots from an image stored in S3 (preferably) or from an image stored on another EC2 instance?
No. iPXE and the more general class of PVS mechanisms require low-level (often Hypervisor) level access to the server in order to 'squirt' a boot image to the VM. This is not available in EC2, and in fact I'm not sure any public cloud service would allow it since a rogue operation at that level could compromise everything on the host.