As I had hoped, this worked out pretty well. The result is much like salsa verde more traditionally made with tomatillos: a green, fairly thin, smooth salsa with a nice sour edge to it.
7-8 lbs green (unripe) tomatoes
1/2 lb green chili peppers and sweet green peppers (ratio up to you)
2 large onions
1 cup lime juice
1/4 cup sugar, or to taste
1/2 tsp ground cumin, or to taste
Salt to taste
Coarsely chop the tomatoes, seed and chop the peppers, and chop the onions. Put it all in a large pot with a splash of water, and cook until nice and soft, 15-20 minutes or so, stirring occasionally.
Puree the vegetables, then stir in the lime juice, sugar, cumin, and salt. Taste and adjust seasonings as needed. Refrigerate, freeze, or can.
To can, use pint jars and process for 15 minutes in a boiling water bath.
Yield: about 6 pints.