There are a bunch of container mechanisms for Linux now: LXC, Docker, lmctfy, OpenVZ, Linux-VServer, etc. All of these either involve kernel patches or recently added Linux features like cgroups and seccomp.
I'm wondering if it would be possible to implement similar (OS-level) virtualization purely in userspace.
There's already a precedent for this - User Mode Linux.
I've seen several projects for userspace tools (though I cannot, unfortunately, seem to remember which ones at this time) which appear to be reusing the Linux kernel make-system, as evidenced by the similarity in build messages:
Thanks for making this year’s plumbers conference such an enjoyable event. Next year, we’re planning to co-locate Plumbers with the Kernel Summit and LinuxCon in San Diego from 29-31 August. The current plan is that Plumbers and LinuxCon would run as parallel but separate events. To accommodate the parallelism, we’re still planning on keeping the numbers for Plumbers down to 30