I have a track I composed as a subcontractor for a jingle company working on a Kodak campaign. My track was not chosen for the commercial, but I like it anyway and I'd like to include it in my demo.
In producing this track I was required to include the Kodak theme melody, as well as the "share the moments, share life" tag, which have been around for years. The track is my unique and original work, but it utilizes my own arrangements of these already-composed elements.
I want to use this track as an example of what I can produce, but I don't want to mislead potential clients into thinking I was the original composer of the Kodak tune. This would be included in a long montage-style demo, so I don't have a track listing in which to include an explanation or disclaimer (also, I'm afraid such an explanation would strike clients as rather unprofessional).
Seems to me in that particular situation, if it was part of a montage of your works and was not for sale, it should be OK. If it gets to the point it comes up with a client, you can explain it then (working with a pre-existing theme for a client). It is demonstrating what you can do under those circumstances. You could not sell it, but you could sell something like it perhaps. Just keep it honest.