I'm reliably informed that wic's rootfs plugin can't create EFI-compatible FAT partitions (I'm not sure how this relates to bug 6513).
I need more details on this. wic can create FAT EFI partitions. Without this EFI images would not even boot. Wic has at least two canned efi images: mkefidisk and mkgummidisk. Both are fully functional as far as I know.
I asked Igor about this on IM: I wanted to use the rootfs plugin to create it the reason is that I am doing something a bit exotic: I do not use a generic bootloader - which is what is supported by default instead i use a blob which includes an efi stub, the kernel and the rootfs the recipes and wic were making too many assumptions that didn't match my scenario otoh the rootfs plugin would have worked, as it simply dumps whatever files it finds but it didn't supprot VFAT filesystem, which is a must for EFI
After discussing this with Igor and Ross we agreed to close it as this functionality is not needed anymore. Feel free to reopen it if there will be a need in it.
verified per above comments.