I’m afraid I’m not aware of anyone else working on seL4 for Hikey at the moment.
The best available description of the boot and firmware code is found here: https://github.com/96boards/documentation/wiki/HiKeyUEFI . It should allow you to regenerate the l-loader and fip binaries if you wish to.
The key question is what sort of loader seL4 requires. Ideally it should support (or be altered to support) being loaded by a UEFI compatible loader. If that is possible then the seL4 kernel could be copied into the boot partition and loaded by grub (i.e. pretty much the same as a PC).