You’re going down the wrong path with this. I know it seems to you that an LOR from a university professor would be helpful, but unless it’s a situation like what @cptofthehouse described above, it is not what the admissions officers want to see.
If you’re only allowed one letter (in addition to the GC letter), it should be a HS teacher who knows you well. A professor you did research with does not know how you are in the classroom.
YOU don’t have to share that your LORs were done confidentially. That’s the responsibility of the letter-writer. Your teachers have done this a zillion times before. They know what to do. The letter is uploaded directly to the college via Naviance or whatever system they use.