Your approach with scheduling a dummy window would be OK except you'll have a hard time extracting the dimensions that actually matter.
What do you need on the schedule? A window is typically WxH, yours is more like W1xW2xHxr where "r" could be the corner radius.
You could try to classify the 3 walls that create the window pane as (3) separate sections of the entire thing and that Element Classification will place them on a schedule with properly defined Criteria. Dimensions of a Wall will not be in the same schedule column as dimensions of a window, because these parameters are entirely different.
BTW, I don't see it rolled, delivered and installed as one piece but I live further West from you...
As far as the plan marker goes, you shouldn't have a problem with some workaround.