Apache implements an eXtended SSI module that allows for the features in IIS's ssinc.dll, plus some additional light-weight scripting features such as conditional processing (if,elif,endif) and page defined variables (set var=abc). This would be nice on IIS.
If you want to advance this even more, the W3C Jigsaw server did some similar SSI extensions including also cookie access, a looping command and counter incrementing command. In addition (and probably too heavy) Jigsaw also implements database (JDBC) access via queries.
The key here is keeping it lightweight, but a little flexible. A sample use is (on the server side) looking at the browser version and optimizing the output for that browser. There are already many scripting options for a full application development (e.g. ASP.NET). Think of this as strictly light-weight control of information display. This is somewhat analogous to the light-weight scripting provided in JSTL (if you could separate it from the whole servlet back-end).
Just an idea.
David