When using unbound I realized it was missing for illumos
So, I started looking for any previous work done on this.
I found one patch that was not integrated, so I synced it with current illumos-omnios sources.
Things seem to be working as unbound is now able to use SO_REUSEPORT, and respond to queries as usual.
More test are needed, and more work is required to integrate this to illumos-gate.
Here is the patch for Omnios: here