This won't take that long. Loadingtime will still be like in a normal MMO game. I would guess max. ~10 seconds or less.
I don't see a problem here.

First of all to your numbers and calculations:
I only know/played one MMORPG in my life, and this was everquest 2. The whole world was split in hundrets of small levels which only contained ~20 players.

1000 or even 100 players at the same time can't be done by an indie developer because of several reasons:
- Such a game won't be played at the same time by so much people!
- You won't have the people to test your system!
- You won't have the money to rent as much servers!
- You won't have the people to create that big world/dungeons...
- ...

And if you as indie developer want to create a MMORPG game do it like the maker of Everquest did it: Split your world into small levels/zones. Then an MMORPG can be done so easy with ANet (read the Multizones HowTo). Now you will say: If there are so much zones, there will be so much level loadings. Yes, thats true! But loading a level with Gamestudio is that fast that this won't be a problem

To the feature itselfe:
If I think more exact about this feature I think about a lot of problems that would come with it.

So my conclution: If you really have a game that will be the next WOW then writing it for yourself won't be a problem. You still will get the richest man on the earth
And for all "smaller"/normal multiplayer games, the current entity manager is ideal!


[EDIT] I'm afraid that I have less time to finish 3DVoIP today. I will post the demo as soon as I'm ready.