here's the content of the INSTALL for this option:
--enable-bannerprog=program - full path to a banner program.
sqwebmail replaces the character sequence [#B#] in HTML template
files with the output generated by this program. The first
argument to the program will be the name of the HTML file. The
banner program can use that to customize banner output.
It is also possible for a site to stick multiple @B tags in the
same HTML page. To distinguish each instance follow the @B tag
with up to 30 letters or digits, surrounded by braces. For
example: [#B#]{TOP} and [#B#]{BOTTOM}. "TOP", or "BOTTOM" (or
anything else) will be the second argument to the banner program.