You can just use Syslinux with the memdisk module (if you want to boot full ISOs stored on the thumbdrive), or you can extract the ISOs into directory trees and

You can just use Syslinux with the memdisk module (if you want to boot full ISOs stored on the thumbdrive), or you can extract the ISOs into directory trees and configure Syslinux menu options to load the contents equivalently to the ISO's original bootloader (as long as they're not using hardcoded paths or volume labels to find the filesystems to mount).