E2B supports UEFI-booting in two ways.
1. Use .imgPtN files
2. Use the grub2 menu system (MBR or UEFI-boot)
As you correctly point out, grub4dos cannot be booted in UEFI-mode. So with .imgPTN files, you need to switch-in the partition before you can UEFI-boot. It does mean that you can secure boot from a wide variety of UEFI OS's - something that no other multiboot USB solution can do!
There are videos about .imgPTN on YouTube and there is a page about the grub2 menu system (last in side menu tab).
grub2 menu systems are not generic, each menu has to be tailored to each individual payload. This means that those solutions need to be continuously updated/maintained.
If you want to GENERIC UEFI-multiboot for a variety of different payloads and want to prepare it under linux, then look elsewhere (and let me know if you find one)!
Subject Re: CSM is NOT UEFI!