You could design a copper heat-spreader to pop over the heat-sensitive part(s) of the board, and then apply a heatsink to that. Maybe copper foil would do the job.
Another approach is to just run the Pi submersed in mineral oil, with liquid convection then taking the heat away from the board and shedding it to the surrounding environment. We usually don't run our electronics like this (even though the general approach is widely used for cooling needs of all sorts) because mineral oil is gross and might even impact the endurance of our hardware - but the Pi is tiny and cheap enough to make this a non-issue.
Another approach is to just run the Pi submersed in mineral oil, with liquid convection then taking the heat away from the board and shedding it to the surrounding environment. We usually don't run our electronics like this (even though the general approach is widely used for cooling needs of all sorts) because mineral oil is gross and might even impact the endurance of our hardware - but the Pi is tiny and cheap enough to make this a non-issue.