[darwinbuild] Roots

Hi, is there an easy way to get DarwinBuild to attempt to fetch roots from a separate location first before the default locations ? I would like DarwinBuild to check my webservers for pre-built roots before defaulting to OpenDarwin's. Also ... while I am here, are gzip'd n' tar'd files the only recognised file type for roots, or can I use bzip2'd n' tar'd files ... or maybe xar ? S a m