As I'm playing with a prototype language and OS while building it on top of seL4, I'm not using any intermediate C layers to connect to the seL4 API (so custom build and integration and no C runtime), while most of the samples out there are heavily relying on some foundations, which is completely understandable, considering how it can be involving to build anything relevant on top of the seL4 API. So as I'm just really getting started to integrate with seL4 (and I love?? it!) I have to go through all these details, but I hope that I won't be spamming unnecessarily here! ?
If you are looking to avoid any C interop or runtime that should be completely possible. The only requirement the kernel makes is that user- level code needs to keep track of the IPC buffer address on its own (unlike previous versions which ensure that a particular register contains the IPC buffer address).
If you want a minimal runtime without TLS you could store the IPC buffer address in a global variable and ensure that your system call wrappers use that global variable to load and store data into the IPC buffer.