Well, F2FS on system partition works, kind of. F2FS reserves more disk space than EXT4 for filesystem internals which pretty much means installation with F2FS is too tight for gapps to fit in (19MB free with test build and no gapps).
System freshly formatted:
- EXT4: 540MB free
- F2FS: 437MB free
I think I will leave it out just because it's not that viable feature to use and will cause confusion if user formats system to F2FS and tries to install gapps (which necessarily won't error out).