It didn't take long for modders to start figuring out a way to get the Russia-only closed beta of Halo Online working elsewhere. In fact, excellent progress was being made to this end until Microsoft caught wind of it and rather hastily killed the project with a DMCA notice.
The mod was being hosted on free source code repository Github, but if you hit it up now, you'll find a lovely DMCA takedown message instead. The actual notice is available for viewing on Github too, if you're curious. It's a fairly succinct affair.
As for a release outside of Russia? Here's what 343 Industries has to say:
Q: Will gamers in other regions like North America and Europe be able to play Halo Online?
A: Right now our focus is on learning as much as we can from the closed beta period in Russia. Theoretically, any expansion outside of Russia would have to go through region-specific changes to address player expectations.