Update
A ready-to-use element for Dart Polymer 1.0 is bwu-bind-html
No you can't bind html using mustache.
what you can do is
<a href="{{text}}">link</a>
with a field like
@observable String text = "mysite.com";
or use something like a <safe-html>
tag - see HTML Tags Within Internationalized Strings In Polymer.dart