Open Source is for hobbyist or "moonlighting" designers primarily. Not exclusively, but primarily.
If a designer has a 9 to 5 job where they can depends upon a paycheck and life's necessities, then they tend to spend their spare time doing the things they want to do which may or may not always be what their employer pays them to do. If they find an open source project they like, it's not a big deal for them to focus on that just to feed their creative beast. This is very common I think. If a designer is unfulfilled at work, they seek other ways to feel creative and fulfilled.
However, if a designer is a freelancer, and putting food on the table requires them to work full days on their own paid projects, assuming they have enough work to keep them busy, then are less likely to "donate" the time to a free or open source project. After all if that time could be spent making money, why would the designer work for free? Now, there are times when freelance work is slow, but when your well-being depends upon it, you are marketing yourself rather than working on projects just for fun.
This is generalizing and purely opinion.
In any case, the only driving motivation to take part in any open source or free project is a desire to use the project, or to learn something new in my experience. No one is going to donate hours of their time to work on something if they don't see immediate reward in some way, and I don't mean exclusively financial rewards. If I use a pre-built Open Source package and I hate the way it looks, but I love the package, I may very well donate time to improving the appearance. If for no other reason, so I can feel better about looking at it.
You mention networking and portfolio pieces. Either of those could very easily be acquired without devoting hours to open source.
I personally see little value in working for free on anything anymore. I'd much rather spend the time making certain I have a house payment next month. And if I am going to work for free, I'd rather help the homeless or volunteer at the Salvation Army or the local Boys Club. Those types of volunteer positions are much more rewarding in the grand scheme of things.
In short, I don't think you can motivate people to participate in open source projects. You can make it known someone is needed and hope there's an interest in the project itself, but much like any volunteer position, it's up to the person donating to be motivated to do so.