Hi Mikele,
If Cornell thinks it's safe, it probably is. Sounds like it could be somewhat messy if it gets wet though.
From another perspective, sunflower kernels have 50-100% more calories per gram and more than 3x the protein than white rice, so while rice may not hurt them, it might not as easily provide the fat calories they need to stay warm during our winter nights.
Please let us know if you see any concerning results if you try out rice. It might be best to start in conjunction with other food types to see what they prefer. And if anyone knows how well the other nutritional (vs. caloric) needs of wild birds are met (or not) by what we feed them, I'd be interested in that myself.
Best,
Greg Levandoski
Longmont, CO