Hi David,
Yes there are some 8 throttle body injection manifolds available for the LS1. Check out the setup on the the new 427 Monaro.
http://www.hsv.com.au/vy/showcars/showcar.asp
Not sure who is making these but I bet they won't be cheap.
Being that I'm such a tight arse I'm going to have a go at building my own. Once I get my motor I pull the intake manifold and make some templates from it. Then I'll make up a plug to cast the rough shape of the throttle body assembly in aluminium. Then the fun begins as I try to carefully machine 8 consistent ports. I've been thinking about using a guilotine or slide style throttle body rather than butterflys. It'll be easier to machine consistently and on WOT there won't be any obstructions in the ports to disturb the airflow. Someting like the system below:
These guys have a setup for the Holden 5.0 and some interesting info about slide throttle bodies.
http://www.oriondevelopments.com/product01.html
Cheers