I was reading up on Battlesystem and when I noticed that they talk how it works with BECMI D&D. Playing around with it I see it working with Swords & Wizardry, Labyrinth Lord, and of course OSRIC. Can work with the D20 as long as you realize that THACO means whatever you need to hit a 20 AC.
If I can figure out the math behind the damage chart I bet I can make a retro clone out of it.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.