Thermal resistance in vacuum
Yes, there's a way: multiply the Emissivity of the package with its surface with the emission of a black body.
(The latter is about 450 W/(m²), at 300 K; emissivity is a factor between 0 and 1; I'm sure your semiconductor manufacturer has a datasheet or application note that says something about its plastics' emissivity)

What would be a reasonable safety factor to use when estimating temperature rise of a power circuit in a vacuum environment?

Do you need that? There's usually both a &quot;junction-package resistance&quot; which is exactly that: the resistance for heat from the junction to the surface of your device. You'd add the resistivity of the radiation to that, done.