It hadn't occurred to me earlier, but earlier today I stumbled upon the fact that Jim Clark actually led the 1968 WDC at the point of his death.
The other occasion that immediately came to mind, of course, was Jochen Rindt, who actually held his lead to become the 1970 champion posthumously.
I also though of Mike Hawthorn, who died in a road accident as reigning, albeit retired, 1958 World Drivers' Champion.
Are there any other instances of drivers being deceased but still leading, be it in Formula One or other championships?