While I agree that seL4 is innovative and exciting, its not an OS or a distro. Its a microkernel, where afterwards, you have to implement your own OS services, hopefully with an exceptionally secure design. (Or just use it as a base for your own unique embedded environment, which is overkill). If you only value your own design flexibility, you'll have an easier time implementing your dream OS in forth, rather than seL4. (If that OS requires multitasking, perhaps one could build a forth environment on top of the microkernel.)
Its kind of pointless to be screwing around with seL4, if you're not some computer science guru, who wants to build an OS with security as top consideration on top of a vetted securely designed microkernel. Its like wanting to use a CNC milling machine to only cut metal pipes in two.
Once you brought up GNU HURD, I then realized you were making an obscene joke. But it would sort of be exciting (not really innovative) in the sense of trying to actually make it do something without crashing. (I half-heartedly wonder if there was anything salvageable from HURD's original designs that would justify the effort to create a MACH emulation layer on top of seL4.)