Is there a way to make the Mac OS X guest OS adapt the resolution of the host OS given that there's no guest additions yet as of the moment?
There is no way for it to automatically adjust, but you can specify a custom resolution for the guest to use.
You can manually edit the VirtualBox configuration, and the boot file in OS X to use a custom resolution. It's a two part (Host VM/Guest) process to enable the resolution.
Here are the steps:
That should help solving your problem.
Note: Install Multibeast 4.6.1 first (registration required)