Hello Stuart,
Thank you for your question about mirroring the UCSC Genome Browser. You can actually do a minimal browser installation, instead of a full mirror as you described. We have a wiki page page, http://genomewiki.ucsc.edu/index.php/Minimal_Browser_Installation, that describes the process needed to set up one these minimal installations. According to the wiki page, the bare minimum of files needed from /gbdb/ are the 2bit or the nib files for the chosen assembly.
I hope this
information is helpful. If you have anymore questions, please
respond to genome...@soe.ucsc.edu.
Matthew Speir
UCSC Genome Bioinformatics Group