Stellarium just got approved today! I was playing with its Oculars extension a bit and found out it was lacking support for Barlow lenses, so after some hacking I proposed it via this branch. You can now find it in trunk, but unfortunately my change came too late for inclusion in new Stellarium 0.12.0, so the only way to test it is to compile latest trunk code, or wait for next official release.
My barlow lenses enhancement allows for adding barlow definitions in the Oculars dialog. Just like in the real world, the multiplicity of barlow lens affects focal length of the telescope, and thus increases magnification and decreases field of view. You can of course achieve this by defining additional telescopes with respective focal lengths, but this enhancement makes it easy to play with different telescope - ocular - barlow combinations without much hassle and duplicating telescope definitions. Enjoy.