Thanks everyone for your inputs, its very much appreciated and one of the reasons we keep coming back!
Went with the pop rivet - I have plenty to spare, its a bit stronger and more water resistant than any glue or tape down there on the muddy side but still pretty straightforward to remove when needed. I don't think there is any intrusion from the rivet end into the shell space, the sideways expansion of the rivet is holding it in place.
Singlespeed is the only speed. I have a devilish grin on my face as I pull off the cable guides, shifters, using the gear wire to form a pentagram on the ground as I lay all the excess fluff from its geared past in the middle and begin the cleanse.
Petrol on the lot and one match later its been sent to kingdom come, the black smog of the burning derailleurs, shifters and cable housing serving its purpose as a signal fire for other singlespeeders to start their pilgrimage towards me, making great headway on the flats but they get delayed on the uphill section which they had to walk to spare their knees.
As they arrive, in their long black robes the festivities can commence and we chant ominously into the dark night: "no gears, no tears", with interludes of sporadic howling and barking.