Another option would be the VirtualBox core combined with PhpVirtualBox. Drawbacks: 1) we also would have to deploy a webserver which PhpVirtualBox needs to function. 2) Running in a browser looks weird. Perhaps somehow it can be made look standalone? ; Advantage: hacking PhpVirtualBox (disabling dangerous buttons) should be easier.
Or we can control VirtualBox through the command line interface only. A (compiled) windows batch file or linux shell script could be sufficient to start the gateway and the workstation, thus hiding most Virtualbox controls.
Might simplify creation of it. Though, maybe then it wouldn’t be called “installer” anymore.
Issue with vbox.me is that it’s unsupported at official VirtualBox forums, VirtualBox developers. Therefore Whonix would move itself into a corner for any VirtualBox issues which could no longer be redirected to upstream VirtualBox.
“Mostly” Portable VirtualBox is something that would be best offered by official. virtualbox.org.