You would only consider this if you were some sort of half wit.
Putting the seatbelt signs on usually requires that the cabin service is curtailed or disrupted as the crew then need to ensure the cabin is secure. It might well occur that one of the crew uses the occaission to avail themselves of the facility, but not the other way around.
Not sure where this concept of the "Captain having to queue up" comes from. I find that a simple "excuse me" with a smile does the trick.