Was peering into the flight deck area (Airbus) on boarding a Lufthansa flight FRA/LHR last September and was invited in for a look by the FO. After a quick chat (asked a genuine question about nosewheel steering coupling) I emboldened myself to ask for the J/S and to my amazment the PIC said OK! Full briefing from FO, gave me a headset so that I could hear the R/T traffic, followed by the full trip stand to stand. Very friendly, very professional. Obviously I observed sterile cockpit procedure until the cruise without being told and it did engage their interest that I was a PPL(B). Dress smartly, ask intelligent questions, accept that a No means just that. I've subsequently flown the J/S with both Go and BA and am very grateful for the opportunities to see real pilots at work!
Most aircrew are rightly wary of being asked directly before take-off - the advice I was given from the BA captain was to let them get to know you via a flightdeck visit and then ask.
Good Luck and tell us how you got on!