No it isn't, unfortunately. It will be in the next beta - hopefully this weekend.
I implemented something, but ultimately wasn't happy with how it worked. I'm thinking of something far more efficient to solve this problem, but obviously requires a bit more work. Basically the proposed workflow is as follows.
1. You create a separate scene, and create a tilemap in there, ticking the Export To Prefab option.
2. Save a prefab into a directory using the newly revealed buttons. This will save the file in there, and create a Meshes directory with the prefab name.
3. In your other scenes, you simply drag this prefab into the scene and use it in whatever way you'd like.
The important thing is that the prefab is just a mesh, and doesn't have any overheads of using it in game (apart from what a normal mesh would). This means that its super efficient to instance these, and all the usual stuff that applies, just works. The source scene doesn't need to be distributed with your game either.
The one downsides are:
* You can't edit your object "live" in the active scene.
* If the atlas changes, you will need to load your source scene and click rebuild all tilemaps to apply the changes. I'll perhaps work on some form of auto-build in the future, but the approaches I can think of now will not scale well with large numbers of sprites, and ultimately scalability is really important.
* In my tests I'm putting multiple tilemaps into the same scene so one click of "Rebuild all" will update all associated meshes.
What are your thoughts on this? Would this suit what you're trying to do?
I possibly still have time to change the workflow to accomodate other things, so this isn't final