I'd like to thank you all that tried to help, but I just never could get the fullscreen mode working as intended. Personally, I think it's because the Crystal Space code that uses XF86VideoMode extensions hasn't been touched since 2003, while most Linux distros moved from XFree86 to Xorg not long after that and the XF86 mode extensions for Xorg haven't really been maintained.
Regardless of the reasons fullscreen mode doesn't work right, I've decided to try to experiment with adding XRANDR support to the CS code rather than giving up on playing PlaneShift in fullscreen. I'm not skilled enough to consider myself a programmer, but I have already managed to get some xrandr test functions working from inside of the CS code.