Essentially, you have little or no chance of being on an airliner's flight deck in flight unless you are airline staff, an official guest of the airline, or travelling with crew.
Unless you now join an airline or suddenly develop a relationship with flight crew, that leaves only the 'official guest' route. I don't think it's very likely to work, but you could write to the cheif pilot of a couple of airlines, explain your position, and ask to view the operation at first hand. If your request wasn't just binned, you could expect a considerable delay while extensive (and expensive) security checks are carried out before anyone lets you anywhere near an aeroplane. Don't hold your breath for this to work out!
Alternatively, some airlines will allow a flight deck visit on the ground during the turnround between flights. This may be your best bet, and can be achieved by a polite word with the senior cabin crew or perhaps a letter to the pilot.
You, I'm sure, will understand why things have changed so much. I do not expect things ever to go back to how they were.