I can't suggest any hints, but I can suggest it will very likely have to be a home-rolled solution.
From the manual,
https://www.virtualbox.org/manual/ch03. ... tossupport :
FreeBSD:
Limited support.
Note that you must enable hardware virtualization when running FreeBSD.
Guest Additions are not available.
Without Guest Additions, there are no Virtualbox drivers for the video card available, so only basic video is available as supported by FreeBSD's drivers.
Oracle adopted the virtual VMSVGA video card, not VMware's drivers. From the manual
https://www.virtualbox.org/manual/ch03. ... gs-display :
VMSVGA: Use this graphics controller to emulate a VMware SVGA graphics device
Installing 3rd-party drivers to handle Virtualbox-provided hardware that the guest OS doesn't have drivers for has worked (I did this once for a network card in an XP guest, and it works for virt-io setups.) Installing VMware drivers for the video card may or may not work. However, even if it does, the Guest Additions trigger won't be there, because the extra stuff that Guest Additions provides isn't generated for FreeBSD, so won't be calling out to the host to let the magic start.
Guest Additions for FreeBSD haven't been made because no (or not enough) Oracle customers want them.