I had been wondering myself, so I googled and ran into the Uniwill forums (duh-why didn't I look there first). The thread below discusses PowerCinema installation, but then goes into multi-OS booting from the P button. I assume that while you can't see the contents of the PowerCinema partition from Windows, you can boot into a DVD distro and edit the bootloader on that partition.
edit: Just reread the thread there. They booted into PowerCinema, and then exited to a shell, where they started the Linux install.http://www.uniwill.com/forums/viewtopic.php?t=38