On the other hand, are there any ideas how to attract more developers to contribute the open graphics stack?
Of course I can only speak for myself and I consider the threshold to enter this "business" to be really high.
To be honest, I wouldn't even know where to start. Reading the specs? Start to understand the available code first?
Lowering this threshold by some introductions and tutorials would be great imho.
Not sure the current devs agree. After all someone would need to write them.
Those blog posts by Alex were worth a read.