Norman, the only reason for loose C 20 nut I see is axial wear at some place in the train of components on the clutch gear shaft when you do up the nut real hard plus a sloppy spline that admits micro friction wear at these places. You are lucky having components with good fit so you have never been faced with this trouble obviously.
I´d be interested in cases where the nut has unscrewed dramatically: What kind of securing means was selected in these cases, safety wire or this wire spring end as designed by Vincents ?? I guess this only happened with very sloppy spline fits ?? --- Seems I repeat myself a lot here . . . .
Looking at drawings of the clutch, the C 20 nut has a ground o.d. for a reason, a bush of the clutch shoe carrier acts on this diameter. Now that bronce bush has practically NO lubrication so what does that mean in the long run ?? Just thinking: Is there a chance with a poorly set/ worn clutch shoe assembly and uneven loads on the carrier that this bush transfers heavy sideloads onto the C 20 nut ? If so, under every hard acceleration there is a high torque load on the servo clutch and with high friction and side load due to no lubrication of the bush there could be a real twisting action onto the nut ?
When no lubrication anywhere in the dry clutch is essential why not better use some lubricant-free bush in this place like some types from IGUS , see link below ? I´d expect much lower friction coefficient and safer operation regarding the loosening of the C 20 nut , Permaglide teflon/graphite bushes another option.
For extending the useful life of sloppy splines I just looked up Loctite types: The 221 seems to be even better than 222 for some joints, same low strength but very low viscosity . Reason for choosing Lotite settling types is the dry compact film can take high loads as opposed to squashy Hylomar that I´d expect to walk out of the joint in time. For assembling all new perfect parts I´d take chain spray plus MoS2 powder for preventing any wear therein. MoS2 can take extremely high loads so great in this application and chain lube will stay there safely.
The VW spline trouble is most likely a bit different as this joint is in motion and does long axial strokes like in front engine - rear drive shafts. These joints require hd grease lubrication for sure.
Vic
IGUS bushes
Loctite 221