Not possible, unfortunately
antipodean : Well, I wasn't thinking that 13
It still wouldn't work because all we have are the digits 0-9, we can't tell the Humax where to put them. e.g. if you replace '1' with 'X' then 11:11:11 would be displayed XX:XX:XX you cant say replace '1' with 'X' sometime's but not allways
BTW
I presume you meant to say '12 would be displayed as XII'
Dekatron counter?
RedEarth : is that a screensaver clock using roman numerals is a bad idea
RedEarth :My preference would be to do away with the clock altogether and have flying toasters instead ;-)