As to the assumption that most would not adopt a cache they'd not previously found: Of course, if the cache has been in need of maintenance with a history of DNFs, often there is no ability to log a find before adoption. I have seen these and thought, "That's such a shame," and considered whether they are in my "maintenance comfort zone", and if they are - if close to my own or my home base - I will consider offering to adopt if it's in a great place or highlights a great feature, facility or service; however, most often the COs have been out of the game for a while and do not respond, though not always.
As to the adoption warning: To me, adoption means transfer of *ownership* and therefore logging a find on one's own adopted cache would fall into the category of PLE (poor logging etiquette). On the other hand, I appreciate that not everybody's thought processes are the same. In my case, I would not expect to be able to log it, but perhaps there are others that might wish to do so. I don't think it's safe to assume that some adopters would expect to be unable to log their un-found adopted caches as a new find.
I see no harm in such a notice ... perhaps a simple, standard checkbox similar to the ones for creating a cache would do. (Whether you're creating a new cache or simply editing the listing, you still have to check those boxes, so I assume there's no code to search whether you've already indicated that you've read those guidelines before.) So, a simple checkbox statement such as, "I understand that I will not be able to log this as a find after I take ownership of this cache," or something - whether the adopter had found it or not - might indeed serve to give notice and pause to some who might expect to have been able to do so, thereby precluding some disgruntled adopters' complaints.
Would it be superfluous? For what I think may be a great number of other geocachers: NO, I really do not think it would be. For me and most who visit the forums: Yes.
edit-spelling, sheesh