It has been frustrating to me that Front Row only plays shared movies that are found in the iTunes library. I couldn't find anything that would work. After reading this hint, I thought I'd give it a try to see if it worked with networked volumes. Lo and behold, it does!
First, it's necessary to mount a share and navigate to the folder that you want to show up in Front Row. Then just make an alias of that folder, drag it into your local Movies folder, and open Front Row. The shared folder should show up in the Movies playlist. You'll need a zippy network for it to work, but it does work!

