It sounds like a good project, Ross.
As for your reasons for not posting it in the CC forums, you needn't worry in this case.
All you have to do is disclose and acknowledge the source of the sky BG. Most of the pic
is your original work, and the substitution process is also your work. Here is the relevant
part of Rule 20:
For competitions, all elements of the pic must be your own."If any elements of the photo are not copyright to the member, this must be disclosed at the time
of posting. Any element added must be copyright free. This applies to using ‘stock’ elements, or similar,
in a photo. Failure to disclose that your photograph includes elements that are not your own original
work will be deemed to be a copyright breach."
As soon as you do that I will shift it into (perhaps) the Photo Manip forum.