How to get VirtualBox guest's name from guest
I have luck by setting the guestproperty in VBoxManage and VBoxControl.
On the host:
VBoxManage guestproperty set SampleVm GuestName SampleVm
Then on the guest:
VBoxControl --nologo guestproperty get GuestName
Downsides are that this requires VirtualBox Guest Additions (for VBoxControl) and an extra call from the host.