You’ll probably get more responses on the Build board:

http://typophile.com/forum/6

Since the space characters are just like any other characters you can substitute WORD + SPACE + WORD with NEWWORD like you would replace f + i with f_i.
But keep in mind, that such ligatures won't work across line breaks. So the typical solution would be, not to address the space character(s) itself, but rather to check for "is-not-a-regular-letter" as defined in you glyph classes.