I used negasilk.ulp for a while for inverted silkscreen, but I decided to enhance it a little and make it easier.
1. First create your art work (Polygon in top layer with text inside it on layer 41 tRestrict with vector font type).
2. Run Ratsnest.
3. Name the polygon with "NEG_SILK" to make it special from other normal polygons.
With this enhancement there are no need to run any script file and also can help you to add negative silk screen within a design has already other polygons.