How would proprietary drivers work on a GPL microkernel like the Hurd? Presumably the driver would be a userspace program. Is there something else that would

How would proprietary drivers work on a GPL microkernel like the Hurd? Presumably the driver would be a userspace program. Is there something else that would prevent proprietary userspace drivers on the Hurd? Seems like this must have been something that people have considered before. Otherwise it would be pretty amusing if GNU/Hurd could have NVidia proprietary drivers that Linux wouldn't allow.