You are undoubtedly right in saying that this would be outside Class Rules,  so that raises the question as to the circumstances in which you might use the rig.   For non-racing,  I agree that it might be fun to try,  although I suspect that it would be not as much fun as a standard symmetrical spinnaker.    For class racing it is an absolute no go,  but you might perhaps be able to agree local handicap racing at your local club;   that would be a matter for negotiation,  and ultimately it would be for them to decide.   They would then need to allot you a suitable handicap,  which might be technically difficult (because of the absence of any hard data on which to base it),  but in the first instance you would need to ask them,  and they might or might not agree.

There would seem to be some technical issues with mounting the bowsprit in the way you suggest.   Attachment to the mast should be reasonably straightforward,  except that on many GP14s it would obstruct the spinnaker pole downhaul.   Attachment to the stemhead fitting might be more problematic,  as you would need a secure and rigid means of attachment while simultaneously spacing the sprit off to one side of the fitting in order to allow adequate clearance for attaching and removing the genoa;   and also adequate clearance for genoa furling gear,  which I suspect you would regard as highly desirable if using an asymmetric spinnaker.    And the combined effect would possibly be to push the end of the bowsprit  –  which is where it matters  –  well off the centre line,  unless you made an appropriate adjustment specifically to correct that.     All these issues are no doubt solvable,  but they would require to actually be solved;    and the traditional solution to the third one is to offset the heel of the mast to one side,  significantly off-centre,  so that the end of the sprit ends up on the centre line.

I agree that setting a conventional spinnaker single-handed is a challenge,  although in lighter winds it can be an interesting one;   I used to do so regularly in light winds until my late seventies,  but now that I am into my eighties I wouldn’t still risk it,  and I never did it single-handed in stronger winds.   A tiller brake helps immensely,  so that the tiller can be kept central while you go forward to set (or hand) the spinnaker.   In my youth I used a pair of lanyards permanently fixed inside the cockpit,  and just quickly tied each of them to the tiller,  but over the last 20 years I have instead used a fixed shock cord across the cockpit below the tiller;   just lift it and place it on top of the tiller when you want it to hold the tiller in place.

For what it is worth,  I have the impression that asymmetric spinnakers are great on a beam reach,  but useless on a run;  so tacking downwind becomes de rigeur if you are using one.   Also it was reported a few years ago that one of the reasons our top youth sailors enjoy racing the GP14 is because it is a very technical boat,  and particularly because of the symmetrical spinnaker,  which demands far more skill than does an asymmetric,  and which then rewards that skill.

Finally,  if you are thinking of modifying the rig,  two further ideas for you.   First,  a few years ago a group of Hampshire sailors fitted out a GP14 hull as a 1/13-scale replica of Nelson’s HMS Bellerophon,  for a local Regatta;   that is worth trying to track down online.   It is possible that I may have a record of the URL elsewhere,  but it is not to hand at the moment;   if I find it I will post an update.

Second,  see the attached photo.   My own A Capella,  sailing in Holy Island Sound in 2010,  wearing the rig of a rather nice American 40-ft gaff cutter,  courtesy of photo-editing software.

Have fun!

Oliver