Let's see. A perfect thermal radiator, i.e, one with an alpha (visible/near IR absorptivity) of 0 and an epsilon (emissivity in far IR) of 1 would radiate 617 W/m^2 at 50C into a dark sky at 0 K. 50C is a reasonable temperature for electronics and electrical equipment, though probably not for batteries. If the temperature had to be only 20C, then it could radiate 418 W/m^2. That's not too bad for a lunar rover, especially as drag would not be an issue. But it does show why a radiator would be impractical on a PLSS that has to produce chilled water at below body temperature.
The sun wouldn't be a problem for our radiator because it's so small that its far-IR flux is negligible compared to its visible/near IR flux that our radiator would perfectly reflect. The warm lunar surface would be a problem, so the radiator should face straight up.