http://www.uesp.net/wiki/Lore:Ansei
Yes I've been reading up on them, perhaps the Ansei could return to Yokuda, obviously slightly weaker than they used to be but they could still be an elite group of warriors capable of summoning Shehai.
EDIT:
Really? How many men are you going to be able to move through there? Small exits, one Imperial would have to stand at the top and stab anyone who climbed up.
The Sewers at their wide points would allow, what? 5 or 6 men across, at their thinnest one man would brush his shoulders on the wall. It doesn't seem practical. Also hundreds of men streaming into sewers would be pretty obvious.
I always thought the sewers were just as wide as the city streets.
"The Sewer System
Underneath the Imperial City is a sewer system as large as the city itself. The sewers can be accessed from most districts of the city; there are also a few entrances outside of the city. The sewer system is a fairly complex maze. The east and west sections of the sewer do not connect to each other, but within those sections there are many interconnections."