I know this question is old but I had a similar challenge dynamically modifying the href attribute that sends an email when the anchor tag is clicked.
The solution I came up with is this:
$('#mailLink,#loginMailLink,#sbMailLink').click(function () {
this.setAttribute('href', "mailto:" + sessionStorage.administrator_mail_address + "?subject=CACS GNS Portal - Comments or Request For Access&body=Hi");
});
CACS GNS Site Admin.
I hope that helps.