We are interested in a source for mechanisms of drug action to enter
into an existing database.  Is there an online downloadable source for
known or suspected mechanisms of FDA approved compounds and/or other
biologically active compounds?  Ideally, the mechanisms would have a
short descriptor suitable for a field in a database as well as
available references from the literature.

Have you checked out Chemical Abstracts yet?  They do list all sorts
of information of any chemical that has been registered and all FDA
drugs should belong to this category.