You can do this with JavaScript, like this:
<html>
<head>
<script>
Prince.addScriptFunc("make_bookmark", function(text) {
var MAX_LENGTH = 32;
if (text.length > MAX_LENGTH) {
text = text.substring(0, MAX_LENGTH) + "...";
}
return text;
});
</script>
<style>
h1 { bookmark-label: prince-script(make_bookmark, content()) }
</style>
</head>
<body>
<h1>This is a very very long heading that will produce a very very long
bookmark unless we use JavaScript to make it shorter.</h1>
</body>
</html>
This should produce a document where the bookmark is "This is a very very long heading..."
