I figured out how to enable this command line parameter, on mac:
Edit Maya.app/Contents/bin/rendererDesc/lightmapRenderer.xml and add:
<attr n="fillTextureSeams" p="1" s='($tmpSet + ".fillTextureSeams")' t="float" h="Set the Fill Texture Seams"/>
after this node:
<attr n="vMax" p="1" s='($tmpSet + ".vMax")' t="float" h="Max V value for lightmap. Used if uvRange is set to User Specified."/>
No the parameter will be taken into account on the command line.