Ok, it seems like they only recognize subtractive notation. Nonetheless, numbers from L onward seem to work, so if you want that distinct "roman numeral" look, you could choose "King LXVII".
They don't check for them, they just forcibly convert them into ASCII by taking the lowest byte of the Unicode code point, turning "ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪⅫⅬⅭⅮⅯⅰⅱⅲⅳⅴⅵⅶⅷⅸⅹⅺⅻⅼⅽⅾⅿ" into "`abcdefghijklmnopqrstuvwxyz{|}~\x7f". A few of these characters get rejected, though curiously not the last one, which is nonprintable.
