Hi Jim,
If it is an element in the filter wheel, it would rotate into the imaging path just like any other filter.

As you may know, with monochrome cameras, at least 4 filters are usually used for each target, a luminance (clear), a red, a green, and a blue. Software causes the filter wheel to rotate a different filter into the light path with each exposure. The 4 exposures can be combined for a color image. The filter changes do not disturb the camera, the telescope, and if the filters are parfocal, it doesn't disturb the focus.

My filter wheel has room for 5 filters. Currently there's a hydrogen alpha filter in the 5th spot. I could remove that. If the mask can be used in this position, a tiny image of the mask could be placed on a clear filter.

As you may also know, unattended telescopes are controlled remotely, either live or through scripts, or a combination. The photographer may be as close as the nearest building or half way around the world. The problem with these masks is that they must be physically placed on the telescope and removed prior to imaging, not a problem if you are sitting at the telescope, but a big problem for a truly remote imager. If one of these masks could be placed on a filter in the filter wheel, the remote operator could rotate the mask filter into the light path just like any other filter, focus with a series of exposures, then rotate the conventional filters into the light path just like usual. It would remove the need for the operator to be at the telescope and there'd be no disturbance of the camera.

David