I think you're going to need X.org installed to take advantage of any of the host integration features.
The only other thing I can think of is to temporarily add the minimal packages
sudo apt-get install xserver-xorg xserver-xorg-core
then install the Guest Additions, and then remove the x packages. However, like I said, I believe you're going to need x windows to remain installed in order to take advantage of the guest additions. If you choose to keep X.org packages you'll also want a window manager like openbox.
Alternatively, you could install the xubuntu-desktop package for a minimal gui in one step.
Update: I messed with this for quite a while and couldn't get it to work without a desktop fully installed. The xserver packages aren't enough on their own to enable the integration features. I also installed xubuntu-desktop on a fresh 9.10 server instance, doing so remotely via ssh so I could log the session and save the list of all packages it said it was installing since removing *ubuntu-desktop only removes the meta-package. I then ran the guest additions install, verified it worked, and finally uninstalled all the desktop packages by pasting the log output from install into a bash script with apt-get remove, followed by apt-get autoremove.
Not surprisingly, it reverted back to requiring a host key like rt-ctrl to un-capture the mouse and keyboard. Even if it hadn't, I certainly wouldn't recommend doing this on a guest you were using for something important. The bottom line is that the guest additions appear to require one of the desktop installations to be present, and while it's true the mouse is irrelevant without x installed, I believe that the mouse integration feature is the only way to override requiring the host key to un-capture control (including keyboard) from the guest.
Having said all that, I think that not requiring the host key ought to be a feature or preference setting for the VirtualBox program itself, rather than the guest additions. If you click on the guest's application titlebar (or on its taskbar entry) to give the guest focus, it gives the keyboard focus to the guest but not the mouse, thereby not requiring the host key to regain the mouse. It seems like there could be a preference setting to act the same way when the guest window itself is clicked, like "never capture mouse". It wouldn't need to know if the guest was cli or gui, just that you didn't want it to suck the mouse in.
Sorry to not have any better answer about this, but maybe someone at
SunOracle will take note and put this in a future version.