In the UK at least, get designers get paid a lot less.
The work they do get paid to do is restrictive and dull.
Designers are generally driven by the desire to create and make 'pretty' things.
Most spend all day in meetings discussing why that shade of pink isn't good for the 99th design iteration of the same icon.
So I highly doubt they would rush to do this kind of work for free.
Most design work open source projects need are quite 'boring' from some designers point of view.
Many open source contributors don't find satisfaction in their day jobs.
The work they get paid to do is restrictive and dull.
Developers are generally driven by the desire to create 'cool' things.
Many spend all day in meetings discussing why that API extension isn't good for the 99th design iteration for customer X.
And yet many rush to do this kind of work for free.
Lots of work open source projects need are quite 'boring' from the technical point of view (documentation, project files, merging, OS integration. etc).
So why do people contribute to open source projects? Because they believe in the product and like feeling empowered.
I've been thinking that maybe a buddy-system might be appropriate for attracting designers.
A: Get some greater insight into the reasons workflow is performed like it is. B: Get some insulation from the "Here is the Github. Go, be fruitful" nature of a lot of projects. C: Have someone assist with VC.
The developer would get:
A: Exposure to ideas about design B: Experience in how to effectively communicate with developers C: Experience in HCI and usability
You, sir, have given me an EXCELLENT IDEA. TO THE INTERWEBS!