Resistance soldering is an interesting idea. Less likely to burn the flux and oxidize the parts that way. You would still need to take care that the parts, in this case, did not conduct too much heat back to the adjacent plastic hull. But it would be less risky than an open flame. I often use a high wattage (150 W) soldering pistol in these small applications myself.
The use of a heatsink certainly has merit. It may require that a little more heat be applied, however. In the electronics world, we sometimes use a hemostat clamped to the work to block the heat from being conducted back. With the bow plane assembly on the Dumas boats, the solder joint is so close to that low melting point styrene hull, you really need to get the technique right the first time; you won't get a second chance.
George "Crazy Ivan" Protchenko
“There are the assassins, the dealers in death; I am the Avenger!”-Nemo
"I'm disinclined to acquiesce to your request; means No!"-Capt.Barbossa