Try playing with the justification settings, reducing the maximum for word spacing and increasing the maximum for letter spacing.
Is kerning the space between letter pairs?
You could look at maximumWordSpacing and maximumLetterSpacing, or some thing like that.
Not sure if it could be done automatically.
I guess you don't want to use hyphenation? When I have this situation, the only way I've been able to solve it is by breaking down the first word in the next line, which usually is a long word. But this requires to check all instances. Definitely not an automatic process.