In part, live rock exists in all tanks in the form of algaes, bacteria, protozoans, and fungi. However, if the 'look' of marine live rock is what you're looking for, therein lies a problem. Brackish conditions at their upper level is considered to have a specific gravity of 1.015. Marine live rock organisms will likely suffer and die in a specific gravity of less than 1.020. There are estuarial plants and inverts that may be used in a brackish setup. However, I don't know of anyone having any great success in maintaining such a setup.