Canon patent application EP3267238 (A1) describes a lens barrel design where you can insert and remove an optical element, i.e. you can insert or remove an optical filter.
From the abstract:
The present invention relates to a technique to insert and remove an optical element into and from an optical path of an image pickup optical system. A first drive mechanism (12) moves a first element (34) in an optical axis direction. A second element (23) is located at an image surface side of the first element and is selectively inserted in the optical path. A second drive mechanism (24) moves a holding member (18) holding the second element in a direction different from an optical axis. A control unit (65) controls the first drive mechanism to move the first element to an object side to a position where the first element and the holding member are not overlapped when viewing in a direction perpendicular to the optical axis, controls the second drive mechanism to remove the second element from the optical path, when they are overlapped.
Being able to insert a filter into the lens barrel sounds pretty cool to me. If I got the patent application right, then the optical element (i.e. the filter) is inserted where the arrow points to in the image at top.