I think I will work on GRUB on SPARC next to address the build issues there.
Before re-enabling COFF for GRUB, we could look into GRUB as ELF on sparc64, because looking at OpenBSD, their second stage bootloader is actually an ELF binary:
```
root@nfs:/srv/nfs/openbsd/6.3/sparc64# file ofwboot.net
ofwboot.net: ELF 64-bit MSB executable, SPARC V9, relaxed memory ordering, version 1 (SYSV), statically linked, stripped
root@nfs:/srv/nfs/openbsd/6.3/sparc64# file bsd.mp
bsd.mp: ELF 64-bit MSB executable, SPARC V9, total store ordering, version 1 (SYSV), statically linked, not stripped
```
...so the sparc64 firmware seems to also support ELF binaries. And according to [6] this seems to work for a lot of systems. If an ELF version of GRUB can be provided by someone, I have a lot of sparc64 gear to test on - though not as many as OpenBSD supports ;-).