Try Flatiron: http://www.texturebaking.com/ It unwrappes you the lightmap UV-Set in a decent way an you can use your mentalray light setup to bake all the stuff. You also can set up a MR sun-system and render it to textures. There are lots of tutorials out there. Iwould use a script because I dont have time to bother a lot…
then what you have is effective packing for one lightmap... you can make atlas when you need lightmaps of different density. that way if all props have their separate lightmap you can combine them to atlas to speed up rendering due eliminating texture switching in memory... this can be done depending on your technical…