Here they raise mint and when they harvest it for the oil, it is chopped into a large enclosed trailer and then steam is forced through it. Then the oil comes out. I don't know what kind of apparatus you could use. Perhaps, you could use a canning juicer/steamer with the three compartments--one for the lemongrass, one for water, and one to catch the oil. We use one to juice berries and grapes on the stovetop.