MegaMan,
I have a few of those that I modified. Basically I took the PAL chip out, and it just has the NTSC proto rom on there now. Completely playable on your NTSC Console. Here's the info from the first run:
http://www.atariage.com/news/7800Sentinel/index.html
I was going to do a second run, but then it became available in a few other places, so I decided to concentrate on other things. Anyhow, if you're interested in one, let me know.