It occurred to me a while ago, that there would be several ways to pipe hot air into the cockpit for use in cold weather. The obvious one is to direct some large bore air trunking into the footwell, picking up hot air directly behind one or both of the cylinders. Another more sophisticated way is to install and oil cooler matrix in the footwell with a fan behind it and re-plumb the oil pipes through it, although, having taken my car out the other day when the air temp. was around 4c. the oil actually didn't get very hot [I've fitted an oil temperature gauge]. There are also electric heaters with built in fans which can be obtained, though, frankly I think the heat output of these would get lost in an open car. Your idea of heated clothing is still probably the simplest, if the output is enough, and I suspect it will be.
Do let us know if this idea works and if your alternator keeps up.
