I am here a little late, but I found this thread while searching to see what all is invovled in retrofitting a scope...
I would be happy to pay $200 to get a zero stop.
Having Nightforce start retrofitting their scopes would obviously be the ideal solution, but being the problem solver that I am (sucessful or not is another topic) I have another idea which I am going to try. I am not sure if it will work, but I am going to try manufacturing a little part that I can toss under the turrets cap. The idea is that after cranking up the elevation, when you spin the turret back down the cap will eventually contact the part and will not be able to rotate any lower. It will take a bit of trial and error to get the part the correct height for my rifle/load and will not likely be precise enough to land me right on "0", but as long as it gets me back to a point below my zero within one revolution then all I will need to do is spin the turret down until it bottoms out and then just spin it up a few more clicks until it is on the first "0". Having to back if off it would not make it an "eyes free" solution, you would still need to look at the turret, but all I want is something to eliminate the need to remember the correct height. What do you think - will my idea work, or will it become an epic failure post in the motivational posters forum?
edit... I actually found another thread on homeade zero stops:
http://www.canadiangunnutz.com/forum/showthread.php?t=330376
It seems that it can be done, but you need to be careful not to overtorque the turret, stripping threads.