I think the kernel provides results in a special memory space where you can read it without using system calls. Performing operations in the kernel on a memory area writeable by user space sounds really dangerous.
It is possible. As you say, not completely problem free and indeed has raised some discussions regarding security implications. AFAICT documentation is scarce (non-existent).
Here is a previous patch prepping for this one, with sample code to both submit/reap events: https://patchwork.kernel.org/patch/10712819/