I have never tried this myself Brian, but I'd approach it this way ...
Shoot the scene with and without the 'faded' subject matter.
Load both shots in to PS CC (caveat: "other image editors are available") on two layers
Align shots
Choose suitable Layer Blend modes until achieving desired result
Adjusting Opacity to get the correct amount of 'see-through-ness'
If Layer Blend mode alone is not successful, consider masking to achieve the desired result
From theory, not practice, whether you're trying to put a dark subject over a light bg, or vice versa, will impact the choice of Blend Mode that should work best - those with experience could probably name them, but I can't.
I guess you could even try from a single image, creating a duplicate layer and processing that to remove the subject (e.g. cloning), then blending and/or masking with original, with reduced opacity to reveal some of the subject.
HTH, Dave
Photographer Joe McNally detailed how this is done in one of his books, I believe it was the "Hot Shoe Diaries:, the method is similar to how you'd use a slow shutter speed to remove people from your tourist shots. Joe had one light source, in his example it was light spilling into the room from the side and having his subject slowly walk towards him. The result was a ghostly image of his subject. It's a matter of timing as too fast of a shutter or too slow movement of subject will create a solid form.
Re-reading your post, you would focus on the scene first, say the twig; then wait for your subject to approach.
Obviously this will take some figuring out. Just for the record this is a single shot with a (for me) exceptionally small amount of pp.
or do it the other way around: take a shot, shoo the fly away, take a second shot. Opening two shots as layers and adjusting the transparency of the top layer is very easy then. This would give you complete control of how transparent the fly looks. If you do a single exposure and shoo the fly away mid-shot, the results will be rather unpredictable.