It is now possible in the latest SVN version of Celestia to override the default temperature and bolometric corrections for stars. New .stc properties are Temperature and BoloCorrection.
An example of this is provided in the latest version of nearstars.stc, where the Sun's temperature and bolometric correction are specified. This removes the need for the hacks currently used to get the Sun's radius to equal 1 solar radius, when it is somewhat cooler than the typical G2V star (perhaps because it is more metal-rich than average).