Isn't a solar panel going to be a poor heatsink, though? It's flat, and thus has relatively small surface area compared to its size.

In atmosphere, yeah, relatively speaking.

But it doesn't matter since in this scenario each chassis is powered exclusively by the respective panel. How hot does a black panel sitting in the midday sun get? That's your equilibrium temperature. As long as it's within the operational limit of the device there's no problem.

The reason earthbound DCs are difficult to cool is because of density. When you match up panels to devices and shelter in their shadow you no longer have anywhere near the same power density.