There is no need for live rock for bio-filtration purposes...
The bio-filtration will begin its own cycle if the system is exposed to a continuous supply of amonia...
Many people are misled with the reasoning behind LR...
Live rock is not essential...I have never bought any of my live rock for bio filtration...I have only purchased it for the life forms which come with it...the beauty of it with the various coralline algeas, etc...
I also purchase it for the purpose of trying to better provide a more close replica of the marine life's natural habitat...
The bio-bacteria will live on anything, (except a quartz cover for a U.V. light)...the glass, substrate, any non moving materials within the system...