About 3 years ago, I planted my onion plants in early April, and the following weekend it got down to about 15 degrees, and they were ok, in fact, that was a better crop than I had in either '09 or '10. They suffered some leaf/tip burn, but grew out of it quickly.

It probably won't hurt them. The snow itself isn't an issue, it's extreme cold that is more of an issue. Can you cover them with a thermal blanket or something?

I think you will be fine. The ground has had a chance to warm up the past few weeks so the frost won't penetrate down the few inches you will plant them.

snow insulates. plants do not die under the snow. The part above the snow might die.

Onions are normally grown over the winter. planted in the fall and harvested in May in the spring.

Some onions will survive and grow over the coldest winters up north. Scallions are like that.

Tomatoes and peppers are very delicate and I have had them survive snow many times. Onions are hardy and tough with cold.

If you seeded your onions indoors and they are not outdoor hardened they might die. If they are purchased plants I would not worry.

Garden dawgie said the main issue.
If your onions are hardened up, the there is nothong to worry about. all plants in onions family can over winter under snow and in frigid cold weather. So do brassica family of plants.