I think an alcohol stove would function, but its effectiveness would be minimal.
You will probably fly to Santiago, then over to Mendoza before you get started on land. Mendoza is the place to get permits and purchase your fuel. It would be easy to find butane blend canisters there, but liquid fuel is much easier and probably more practical high on the mountain. That is especially true if you get a multifuel liquid stove, because the quality and nature of the liquid fuels that you buy there are not perfectly controlled.
I remember that we were directed to a hardware store near the permit office, and we went there with each expedition member carrying two empty liter-size Sigg bottles. The hardware store man said that he had "gasolina blanca" for US$1 per liter. So, we handed over our bottles, and he brought them back full from the bulk drum in the back room. We paid and left. After we had walked a mile back toward the hotel, one of the guys asked how we knew what we really had in the bottles. We did a test burn that evening, and it burned hotter than the white gas we knew from home. Virtually everybody on Aconcagua was using white gas, and I can't imagine that it would be that much different on Mercedario.