Something like Coleus would give you a fast coverage, and they actually might compete pretty well with the roots of the hedge plants too.
The hedge is looking a little bit better. Did you thin the tops somewhat? Keep it up. The plants will branch gradually lower and lower if you keep thinning out the bushy, stunted branches up top.
(my hands are just itching to come and help you thin out that hedge, wish I lived closer to you)
