Very interesting question. Unicode specifies that the break should not happen within a number: http://www.unicode.org/reports/tr14/
And it's discussed more here in a webkit bug report: https://bugs.webkit.org/show_bug.cgi?id=108347#c22
So I think the answer is that CSS explicitly does not support what you want because Unicode says that it shouldn't. I have hit very similar constraints in the past. In that case I had to parse the string to insert a Zero-width space between all characters. That would work for you if you have the ability to modify the number string like that.
When you set word-wrap:break-word
and add the zero width spaces you get this:
(Maybe someday they'll add word-wrap: break-ALL-i-am-serious!, but I wouldn't hold my breath for it.)