Yes, this would be nice to have this.
I recently got parts of the VirtualBox Guest Additions (for VirtualBox 4.3.12) working in Tiny Core 5.4, but lost it after a reboot because I didn't know what files and links to keep in order to make an extension.
The parts of the guest additions I am interested in are the shared folders (to access Windows drives from the Tiny Core guest), and the shared clipboard. Both of these features worked by running VBoxLinuxAdditions.run from the CD, plus running some scripts, though there were some other issues along the way.
The mouse works well anyway with my current Tiny Core guest. It only started working properly after installing Xorg.
I'm not working on the Guest Additions right now. It should be possible to get some useful parts of it working quite easily, though.