I have done (and am doing) it.
It is a bit complicated since unRAID v6.
You have to make a real USB drive (same as you will for regular unRAID) and pass it through to the virtual Machine.
BUT !!!! ...
In addition you need to create a VDI disk which will be mounted as an IDE drive (master) and will be identical to the USB drive.
unRAID will start booting off of the IDE drive - there is no way of USB booting in Virtualbox - and then once the operating system is copied into RAM it will continue to load from the USB drive that you passed-through.
If in the future you need to upgrade the unRAID OS you will need to change the files on the IDE drive.
BUT !!! ....
all the unRAID settings, ie: plugins, logs, etc... will be located on the USB drive.
You will be able to run it for only 2 months with the test license.