The reason it's hard to find a plug-in for this isn't that it isn't very "jQuery"ish. By that I mean that jQuery plugins normally operate on jQuery objects and the selection has nothing to do with any elements.
Edit: I missed the fact you posted this link in your question but I'll leave it below for completeness since my version is formatted better (but otherwise identical). :)
<script language=javascript>
function getSelText() {
var txt = '';
if (window.getSelection) {
txt = window.getSelection();
} else if (document.getSelection) {
txt = document.getSelection();
} else if (document.selection) {
txt = document.selection.createRange().text;
} else return;
document.aform.selectedtext.value = txt;
}
</script>
<input type="button" value="Get selection" onmousedown="getSelText()">
<form name=aform >
<textarea name="selectedtext" rows="5" cols="20"></textarea>
</form>