But there is a "right" way that it could be done: At OP's request, DL theoretically could contact the mystery gentleman and provide him with OP's contact info and brief message ("I was the person seated next to you on this flight; please contact me if you'd like to continue our conversation"), which he would then be free to act on or ignore. This is SOP in any professional situation when someone asks to be put in touch with a third party to whom privacy is owed: The one who seeks to initiate contact is the one whose info is shared.

AFAIK, doing so at OP's request (without revealing anything to OP about the pax) would not violate DL's obligations, so it is "merely" a matter of making it worth DL's while. If OP really wants it, perhaps a business attorney who has airline industry contacts and experience negotiating one-off deals could help. Airlines, like all companies, presumably make lawful private deals all the time if the price is right.

There is some $ offer amount ($100K? $1M?), put in front of the right office at DL, that would have them sit up and take notice of the opportunity to pad the company coffers by humoring OP, assuming that the request is lawful. That is, large enough that they would fear being dinged by their bosses or shareholders for ignoring the revenue opportunity. Unfortunately this is probably more than OP is willing to spend.

Another option is to hire an experienced private investigator to see if there is any chance that more clues can be gathered through public records and voluntary interviews, starting from whatever OP does remember.