You can make arbitrary changes to static documents by configuring an Action which launches a CGI script. The CGI is then responsible for setting a content-type and delivering the requested document (the location of which is passed in the PATH_TRANSLATED environment variable), along with whatever footer is needed.
Busy sites may not want to run a CGI script on every request, and should consider using an Apache module to add the footer. There are several third party modules available through the Apache Module Registry which will add footers to documents. These include mod_trailer, PHP (php3_auto_append_file), mod_layout, and mod_perl (Apache::Sandwich).