You can have express handle SSI with the help of the ssi node module.
I put together a github repo with this simple example: https://github.com/sfarthin/express-ssi-example .
I deployed this app to heroku so you can see it in action: http://intense-basin-9464.herokuapp.com/
app.use(function(req,res,next) {
var filename = __dirname+(req.path == "/" ? "/index.shtml" : req.path);
if(fs.existsSync(filename)) {
res.send(parser.parse(filename, fs.readFileSync(filename, {encoding: "utf8"})).contents);
} else {
next();
}
});