I still think it should be a default of 10m, but course designer/TD can adjust that if designed into the course.
You could have oval shaped "putting green", through either rope/paint (temp at tourney) or through course vegetation/cutting/planks/rocks/wood chips etc. (i think idlewild has a couple holes with this , but not related to this putting stance rule)
Or hourglass shaped greens. or square, or off centered, whatever. Create some difference and uniqueness to the "Green" at each hole. (as time went on, new courses would have this design element built in to it. Or old courses could be redesigned to incorporate this. That way it wouldn't all be paint/rope.)
Basically, if you are within the confines of "the green" as designed on that hole, you cannot go past your marker. If you are not within the confines, you can. (obviously discretion would be needed so as not to make the WHOLE course "the green")
There is a course by me with some really cool mowing into almost "islands" at each tee and basket. I might try it out next time I play and only allowed to go past my marker outside the greens. There are a couple holes with pretty large greens, and a couple with tighter (based on length of hole) So it will really show some differences on some longer putts than 10m where i can't go past my marker. See if it makes any sense out on the course, instead of just in a discussion forum.