I am currently designing a drop in filter to be used with Nikon Full Frame DSLR's. The reason I am doing so is due to being unhappy with how the Optolong drop in filter sits. I also wanted to use Astronomik and other higher quality filters as well as narrowband.
Currently the cell sits in really nicely and still gives access to be used with many lenses without the lens bayonet hitting the filter cell. I have cut up an Astronomik CLS CCD filter which is currently sitting in the cell.
Currently I have tested the Cell with the following lenses...
At 300mm and above there's always the drop in filter holders built in to the various Nikkor telephoto lens bodies like the 300mm f2.8, 400mm f2.8, 600mm f4 etc...., but they use 39mm and 52mm filters respectively so there's still some finessing required for use with typical 48mm Astro filters, but the mirror is free to move.
I know exactly what you are talking about because I used to use a filter in the 52mm cell on my 300mm f2.8 however I wanted to use the filter with smaller focal lengths hence the design of my filter cell.