the original L4 is not suitable for building object-capability systems
The original l4 doesn't have the virtual layer either, another case of things not coming at the right time.
I understand rump you will get drivers, filesystems and fun but it's sad they downgraded back to mach, there's a superior design pattern with l4 which allows faster switching and 'lean modular drivers'. Not that anything has ever used the latter in practice even though it's inherited from the parent project, then becomes code rot. Rump is monolithic from this but better than nothing, mach doesn't have to be slow but no promises with the low development resources here.
Personally, I've had a dream to start a Hurd VM and hack around with some hardware drivers within Hurd.
Start with netbsds rump if hurd's rump interface really is the same, making uml operate like rump seams like a bad idea, uml is always broken.