if I correctly understand your question the need is to put the VirtualBox server on the thin client in order to (I imagine) execute many virtual machine on the thin client. It is correct?
Currently the plan for ThinOX 10 do not foresee the installation of the VirtualBox on the thin client so I’m sorry for that.
But just to better know your needs: which type of virtual machine you need to run on a thin client (e.g. Linux SO or Windows SO? Which type of resources, ram and disk, do you need for your planned virtual machine?)? What is the real purpose of that? Maybe we can find a different solution or approach.
Sorry for all these questions but I want really understand your needs.