If you're interested in contributing to a POSIX-compatible seL4-based OS, you could take a look at my OS project, UX/RT https://gitlab.com/uxrt/. It will be a QNX-like OS written in Rust and C, using a fork of feL4 as its kernel binding library, along with ports of a few of Robigalia's mid-level libraries to feL4 (the Robigalia kernel binding library doesn't use libsel4 and is broken much more readily by kernel changes than feL4, which does link with libsel4).
Currently I'm not very far along with it though. I have a bootloader (UX/RT will use a somewhat QNX-like single image boot architecture, unlike any other L4-based OS) and build infrastructure, along with a completely nonfunctional root server that prints some of the bootinfo to the serial port, as well as the aforementioned library ports. I just finished porting Robigalia's allocator to feL4, and I am about to start writing code to initialize and test it.
I've got most of the design planned out, although I'm still undecided on some of the specifics of the implementation. My (not particularly well organized) notes on what I've got planned are at https://gitlab.com/uxrt/uxrt-toplevel/blob/master/architecture_notes.