A more over-the-top (brute force, actually) method would be to go fully above 3dsMax, directly capturing the screen with .Net. Navigating 3dsMax handles, you could get every viewport Window and their coordinates in screen space, and then capturing this specific region.
Here's two perfect references, WindowShopper by Rotem Shiffman, brilliant stuff :
This will help you visualize the flow and navigate the software hierarchy, plus its code is non-encrypted, so you can get back every method he used.
You'll get something pretty much like this, that's for the coordinates part :
Then, for the screenshot itself, I'll let you check this Stack Overflow's thread :
I'm aware this looks like an overkill (and might be), but this just popped up in my head so here it is. Good luck