A simple sewer corridor should be easy enough to do. For me, object shadows often proved to be the most time consuming; dimly lit sewers are easy. Maybe even I'll do it.
The traps should be deadly enough if Husam himself is afraid to go in there. I know riddles are over-the-top... it was just an example of taking it one notch up too many. I'm doing Durlag's Tower right now, so maybe I'll get inspired.
The whole idea behind it is that a lot of players never even meet Husam, and some don't even realize there's a palace basement with its own movie. So, the rationale, here, is that you will want to choose the sewers to get the Thieves to help you out in the fight. Otherwise, they won't ever make it on their own. And they don't have invitations...
Once you emerge from the basement, the ceremony actually begins and everybody is focused on the speech. Then Sarevok (appropriately surprised by your showing up) reveals his plans and there isn't really that much time for guards to react to your presence.
For convenience, the guards in the basement could end up silently-assassinated, should you choose that path. After all, Sarevok mentions a group of gangers going through the sewers in one of his letters. If you use the invitation, nothing changes in the basement, and you can't enter the secret passage from there to bypass the traps and grab exp from the assassins.
Edit: I'm making the area. I also have a cool idea for an interactive dialogue-based ward/trap. Of course, it's going to have something to do with sounds.
I don't want to introduce something that obviously can't be considered a restoration, so as a compromise I've decided to drop the ambush idea. After all, I want an alternative to the fight with the assassin duo. A new area is still too much to be part of the Unfinished Business
, but I still want to try something like that in modding.