Don,
 3 possibilities:

 1) You can now get the minimum topography grid values through http://maps.fsl.noaa.gov/MAPS.domain.cgi. (This was just added this morning.)
 2) You can calculate the pressure at the minimum topography by combining the temperature and potential temperature.
 3) If you have the native grid data (bgrb*** files), you have the theta-v (virtual potential temperature), q (water vapor mixing ratio), and p (pressure). You can devirtualize using q and depotentialize using p, and then you've got the temperature at any native level, including the model surface.

Hope that helps.
Stan