Proprietary drivers already exist and they are legal. (Whether or not that's a good thing, is a different, controversial topic.) They work by having the user co

Proprietary drivers already exist and they are legal. (Whether or not that's a good thing, is a different, controversial topic.) They work by having the user compile an open-source shim module which allows device control from userspace. Then the user installs a proprietary program into userspace (i.e. as the root user) and this program permits control of the device from the rest of userspace. There is nothing explicitly stopping a user from installing a different, open-source program that talks to the open-source shim module.