We've made some progress on this topic.
I have available a list of our FAST subject headings. They are derived
from LCSH and may be of some use. The folks that produced this file are
working on producing a similar file for LCSH.
The file can be found at http://orlabs.oclc.org/FAST/fastOutput.zip.
The file contains tab delimited records. The first column is the ID
number of the FAST record that the term comes from. The second column
is the MARC Authorities field that the term came from. The third column
is the term itself. The 4th column is the count of records in WorldCat
that are retrieved when that term is used in the Keyword index. The 5th
column is the count of records in WorldCat that are retrieved when that
term is used in the Subject index.
I expect to have an AutoSuggester built on this today.