You could if you knew which menuentry Ubuntu was in its grub.cfg file. In all likelihood, it is the first. You would also need to know which entry xp is. You would also need a timeout set for Grub, the standard is usually 10 seconds. Then you could arrow up to the Ubuntu entry, hit the Enter key and have grub boot Ubuntu. If there is no timeout, this obviously won't work and you will have to get into something more complex.
I'm not sure why you would have the bootloader of one system default boot to another system which is incapable in a default installation of writing to the other system, or even reading or recognizing it. I've not used WOL but, from your post I expect you don't see any menu?
The suggestion above of editing the grub.cfg file should work but if you don't make the same edit in /etc/default/grub, it will be gone after a grub update. I guess that won't be a problem though if you can't boot it?
Last edited by yancek; 07-06-2014 at 06:39 PM.