Could you create a sketch (2D or 3D) and place a driven dimension for the offset, then create an equation to set your offset distance?
Add whatever construction geometry to your sketch to allow you to add the dimension.
Might need to to a couple ctrl-q rebuilds to get the surface offset to update.
Maybe I'm misunderstanding you, but the surface offset feature itself doesn't seem to accept anything but a number in its blank, no equation, no dimension name, no relationships. That's what I'm struggling with.