is the background color able to be changed? the white background is causing white edges.
if it has to be hard coded, white is probably the worst choice. mid grey would be better.
It is possible to override the settings sent to Beast by directly editing the XML file Unity sends it. If you click on the little drop down menu in the upper right corner of the Lightmap Editor window, there is an option to “Generate Beast settings file”. This will create an XML file in your lightmap folder. You can edit it with a text editor and change the background color Beast is ordered to use.
I haven’t tried this myself, but it is supposed to work.