We've been using properties and labels to get this information. But the intention here is to obtain that result in our documents indeed. We generally deal with 'Masonry Projects' from other firms, and having the rough wall showing correctly would improve the accuracy in the process.
Still would be nicer to have the gap actually reducing the rough wall representation, so the masonry blocks can conform exactly to the Archicad walls. We'll stick to our property method for now and give the window marker gap a try.