I've just started using OD Plus and have not found an option for this. Maybe it was felt that everyone would have Icons of programs to fill the space that it wasn't needed.
Yeah, my set of CS3 icons fills the space nicely... The would be another good option though to enable the icons to be evenly spaced to always fill the tab. So if you had three icons one would be all the way to the left, the next in the middle and the last all the way to the right.
[_____________]
[x_____x_____x]
[x__x__x__x__x]
[x_x_x_x_x_x_x]
or
[_____________]
[_____xxx_____]
[____xxxxx____]
[___xxxxxxx___]