Clutch springs are something I'd not considered, 32 year old springs will have lost there tension, any slight slipping friction would increase the heat at the contact point of the operating arm transferring it to the clutch fluid.I was thinking the clutch springs. They weaken with time.
They will cause the clutch to slip. With the extra weight of the sidecar it might be the tipping point. When slipping, the clutch might overheat the Slave cylinder.
However channeling or ducting air down there is a great idea and incorporating a "heat shield" in the process is a better one!!
As a temporary measure I may try to fabricate some form of air scoop as suggested, the hack is hung on the left side and I have stainless Motad exhausts fitted so the heat from them can be quite high in any dead air zones. just been for a short ride to gas her up and all is well, come Friday I have anther 150 mile trip to make however the hills are not as steep in the reverse direction so I'm not expecting any issues this time.
There is a 90 deg right hand bend at one of the gear change points, I take the corner at around 55-60mph, but as the master cylinder is full and the bike remains upright, I can't see that the delivery hole in the master would be uncovered, the air is always at the slave cylinder end, right at the bleed nipple. If I had the time I could stop as soon as this happens and just vent off but that's difficult at 0430 in the morning.