virtualbox.org

Has anyone tried to slipstream the guest additions drivers and service directly into the Windows install? I think nLiteOS could do this. I've searched around, perhaps not well enough. Does anyone have any thoughts on this?

I've done that once. Not a good idea. Two reasons for this.
1: updating to a newer version could be a problem, uninstalling the old one which you slipstreamed will not be possible.
2: Not all features work. You won't have Shared folders, timesync and dynamic resolutions. Yes, you do have higher resolutions, but not changing dynamically, because that's done by the service that comes with the tray icon.

So, it's possible, but you will have only half of the functions and you end up installing the rest anyway.