Home at last and after testing I can add more characters to the font10x7[] definitions and then alter the 0x7f bound in loadBufferLong to display them. I'm not using the wordclock to display though (one is 400 miles away and the other my wife won't allow me to play with any more) but am using a Rainbowduino with the same core code as used in the clock, just the rendering routine is different.
I knocked up an extended (0x20-0xff) double width font based on the Windows Terminal font while at work but it's now 12x8 instead of 10x7 so will need a few more changes to get it working + not all the extended characters are correctly defined. I also had the original (pre double width) 6x8 version but unwittingly messed it up when doing a final search/replace to make it arduino compatible. I will do it again when I go back to work next week.
I used a free GLCD font creator from here to import and convert the terminal font and then knocked up a quick VB6 program to change the font program output from hex to binary and rotate the data through 90 degrees, another VB6 program I had already was then used to double the width.