What has been reported to work is to buy Windows 10 license and then you can use the machine on bootcamp as well as via VM.
More details here: https://communities.vmware.com/thread/516838
I got the same error you linked until the latest rollup fix (which put my install into a boot loop due to nvidia geforce creating a phantom user but that's a whole different problem).
Once I installed the rollup it gave a different error and then took a day or two for the activation servers to finally work and activate the install. I only run windows natively to play games so it only runs for 2-3 hours at a time.
It took until yesterday to get it all sorted out. I installed win10 within a few hours of the iso going up.
Thanks for the heads up.
If possible please also post this at the forum thread that I referred to.
But it was tricky to build that VM. I followed instructions from [0], but don't remember if I had any troubles. Even if Windows itself when running in the VM, MS Office complains that it's not registered - I guess MS Office uses a different method to determine if it runs on the same hardware. Was not worth to bother for me because I have to use Windows only short times from the VM - and if I have to use it over extended periods I boot directly to it. But it would have been slick if it worked 100%.
[0] http://geekery.amhill.net/2010/01/27/virtualbox-with-existin...
Even trickier: on a Mac using VMWare Fusion, you can have a Windows partition that you can both boot into and run as a VM within OS X.