Today, just to see what would happen, I downloaded a Windows Server x86 32-bit Install image (X13-04874.img Standard Edition Eval version) , renamed it to X13-04874.iso and placed it in the \_ISO\WINDOWS\XP folder on my Easy2Boot USB drive.
I then installed Server 2003 to a SATA hard disk on a Virtual Box VM using the DPMS2 XP drivers and it worked fine. The second CD had to be extracted onto the flash drive and then I ran Setup2.exe to complete the install.
It may not work for all systems as sometimes there are separate 2003 drivers required, but it seemed to work fine on the VBox Virtual Machine. Of course, unless installing to IDE disk drives, the 64-bit version of 2K3 would probably not work and most people would want the 64-bit version. E2B does not support XP 64-bit mass-storage drivers (and before you ask, I have no plans to do so!).
P.S. If you have a working USB Flash drive that will install Server2K3 x64, then you can convert it to a .imgPTN file and add the .imgPTN file to E2B.