With the Creator's Update of Windows 10, you can now have multiple partitions on a USB drive and WinPE will recognize them. This will be heaven-sent for large WIM files.

The scenario would be to format the first (bootable) partition FAT32 of around 2GB so you can boot UEFI. The remainder of the drive could be NTFS and contain the image(s). I've manually tested this and it works like a champ. Now, if we could just get the ImageAssist tools updated to handle it so I wouldn't have to split my WIMs anymore. :D

Thanks for listening!