Custom sculpt map sample area?
A quick thought an over ride to to define the Min Corner and Max Corner at which the sculpt will be created with in.
The reason behind this would be to allow you to basically change the center of the sculpt. so the person for example would define <-100,-100,-100> and <100,100,100> and they would build everything within <0,-100,-100> by <100,100,100> this would then put the center to the very side of the build. Of course would have to check if the persons build if it goes out side the defined boundry and give a warning.
Right now I use photo shop and use sevral methods of doing this but is very time consumming.