I think, as long as your relish is ok to waterbath, that it shouldn't be a problem. There was this post years ago about subbing tomatillos for green tomatoes.

My chart says tomatillos have a pH of 3.83 and tomatoes have a pH of between 3.5 and 4.7. If your recipe is safe to can with green tomatoes I see no reason why it wouldn't be safe to can with tomatillos, especially given that it is a relish with added vinegar. It would depend more on the amount of low acid vegetables, i.e. the peppers and onions in relation to the amount of vinegar than it would the switch to tomatillos. So, if you are confident that your recipe is safe, I'd be confident canning it with tomatillos.

Happy canning!

Annie

Thank you both for your comments! I was pretty sure it would be safe, but it's nice to have some expert opinions!