...to make it slacker/lower? Seems easy enough given the position of the upper shock mounting area. Drill a new hole just above the existing one? Maybe above and a little for or aft of the original?
Cycle the frame with no shock and see where the contact point is. Might be the tire to seat tube also. Then measure your eye to eye at that point. Figure out the length of the bottomed out shock and those two measurements will tell you how low you can go. Anything less should be kosher at that point. Sounds like a fun experiment.