substituting cuts of meat
By pork sirloin I'm assuming you mean a pork loin, and by pork butt I'm assuming you mean a Boston Butt, or the upper portion of the shoulder.
In theory, yes, you can substitute. But you should think of it this way - would you substitute ribeye steak for flank steak? Again, in theory you could, but the dish will be different. A Boston Butt has more fat and connective tissue, and the meat itself is both tougher and more flavorful than a loin. With a shoulder cut, the general idea is to cook it low and slow, to an internal temperature of around 195F, and the fat and connective tissue break down the fibers, resulting in a cooked product that has lots of pork flavor, and is fall apart tender.
A loin, on the other hand, is a lot leaner, and has a little less flavor. It's meant to be cooked to a medium-ish temperature, which means taking it out (if a whole piece) from the oven at about 145, and it'll keep rising to about 150. Any higher internal temp than about 160 in today's pork loins will make it dry and tough.
So the answer is, yes, you could substitute. But if you cut up loin and cook it for many hours, you'll over cook it, and you'll end up with tough bits of meat. In addition, I don't think a pork loin (at least a conventionally raised one; a heritage breed is far tastier) would hold up to the strong flavors in a chile verde. I mean, you'll have meat in there, but I'm not sure it would be clear that it's pork loin.
If you're making it for yourself for quick lunches, and you have a loin you need to use, sure, go for it. I've done that in the past - use a sub-optimal cut when I know it's only for me, and it needs to be used before it gets freezer burn. But if you're having guests over, I'd go to the butcher for a bone-in Boston Butt, cube it yourself, and throw the bone in the mix for some added flavor.
How about roasting the loin, and serving it with a salsa verde? In other words, work with the same basic flavors, but cook the meat in its optimal manner. I prefer to think of what's the optimal thing(s) to do with the ingredients on hand, rather than the optimal ingredients for a given dish.
A portion of the loin could used as the base for the chile verde