I'm not sure if this is something you're still looking for or not, but the main issue like you stated above is that UEFI doesn't have drivers natively to read NTFS, and the custom wim image is too big to put on a fat32 partition. To overcome this you can use a USB drive that shows up as a fixed disk (WD passport drives for example), and then setup 2 partitions, the first one a fat32 partition with winPE and a second NTFS partition with your custom image. You then boot into the PE environment and have it apply the custom image from the NTFS partition since PE has the NTFS drivers needed to read the second partition.
Here's a link to a blog post that goes over the details on how to setup your drive:
If you use the free Microsoft Deployment Toolkit (MDT) it will generate the PE environment for you as well as making it much easier to manage any changes you want to apply after imaging to the system (I highly suggest using MDT if you are doing a lot of imaging).
Doing it this way will allow you to utilize your custom image while leaving secure boot on.