If it was a pitch can you not just say so?
I see a lot of agency sites that have concept work listed in their portfolio. However this is probably with the consent of the person/company the work was pitched to. Don't be so defensive and try to work a way round it. Ascertain their reasons for not wanting it there but also point out how important it is for you, as a young designer, to have a good portfolio of work. Express how much time you spent on the design etc.
That's my sensible answer.