Actually I built my own RGB-NTSC converter board. I'm using one of the latest NTSC converter chips. I was able to build a prototype board for only about $15. MAS systems charges $125 for their boards, which I think is extremely high. Besides, I heard the composite video output was pretty bad on the MAS board. I understand why the composite video is bad on most converter boards and I know how the proper solution to fix it (explanation is complicated).
My circuit board is complete, I'm just waiting for the encoder chips to arrive.
I don't really need a home mvs system, but I just thought it would be fun to try to build one from scratch. I'm an electrical engineer and I enjoy these types of projects. I was thinking about possibly even selling the boards if I am happy with their performance. I will probably just end up making a tutorial for making the boards and posting it on the web.