Is there any way to add bookmarks as external links that are NOT smart tunnels and NOT rewriten urls when using the VPN Portal?
This is covered briefly in the ASA Command Line Guide. By default, all clientless traffic will be rewritten. You can use the content rewriting feature to disable rewriting for certain resources. I have included a link to the feature for your reference. In ASDM 6.2, you can find this under Remote Access VPN->Clientless SSL VPNAccess->Advanced->Content Rewrite.