This is a request for VirtualBox, a virtualizer for x86 hardware. So if it was made portable, you could run your favourite operating system anywhere.
Now, seeing as John is so amazing at writing launchers, and he says he can't release the portable QEMU package or whatever he says he's created. What about the open-source version of VirtualBox, i haven't tried it, mind you, but it's all the same, just doesn't support the additions.
I don't think making it portable would be too hard, well probally still pretty hard, you would just have to adjust the VirtualBox settings to have the .VirtualBox folder somewhere else, then it may be possible for John to write a launcher that xml files to direct to the drive letter VirtualBox is being run from, so it would correcly find all of your virtual hard disks, vdi, etc.
There's more to it, but have you ever been considered? I know it's been 'kinda' suggested, but I've seen John do so much with drive letters, i don't think it would be too hard, I don't know how NSIS handles xml files though.
So...possible? Or some major downsides, catches, etc?