... tricky. There are hard-coded Settings for the Admin Panel too.
But I think I figured it out:
The corresponding HTML is
The URL in window.open is being replaced by
<div id="ZLoginBannerImage" class="ImgLoginBanner" onclick="window.open("http://www.zimbra.com","_blank")" style="cursor: pointer;"></div>
So if you change line 260 or so:
params.companyURL = ZaAppCtxt.getLogoURI () ;
ZaSettings.LOGO_URI = "http://www.zimbra.com";
it should work as intended. Maybe you need to rebuild the Skin Cache or restart Zimbra. Will try later and keep you posted.
Remember the Logo Licensing Rules: Only if you are using ZCS under the network (commercial) license, you may use your own logo and URL in place of the Zimbra logo and URL.