Well, here's a relevant commit that shows what I did in my last attempt: https://github.com/fholger/thedarkmod/commit/87880984c894fd2dd68ba87e8bf34785ff5eaf1c
Note, this is almost two years old, things may have changed. I also used TBB at the time instead of porting BFG job queues for easier development. You'll see that I had to replace a number of the allocators because they are inherently not thread-safe, and use atomics at several places. These are not light changes to be made; even though it worked fine in my tests, they may not be safe. That's another reason why I abandoned the effort.