I was thinking about making some too if I can find a piece of brisket smaller than a surfboard....
Here are the three very different recipes I found: the first doesn't use sugar so I'm assuming you could just leave it out altogether. The 2nd has sugar and the 3rd only uses 2T. The first recipe gets its nitrates from celery, which must be better than saltpeter.
Home-cured Corned Beef | Nourished Kitchen
How to Make Corned Beef Brisket, Nitrate-Free at Home | Jun-Blog
Pickl-It: Healthy "Gray" Corned Beef