There's a place in Santiago that is famous for having lots and lots of casas de cambio, which is located on Agustinas near Paseo Ahumada, about 2 blocks south of Plaza de Armas:
Along Agustinas, you will find lots and lots of places that exchange money:
If anybody in Santiago is going to buy Paraguayan Guaranies, you'll find them there.
Unfortunately, the day I was in the area to take these photos was Sunday, so everybody was closed up for the day (oh, by the way, almost everywhere in Santiago is closed on Sundays), so I wasn't able to stop in and ask.
An alternative approach would be to go on traveler and expat sites like Couchsurfing and InterNations and see if anybody in Santiago is traveling to Paraguay and wants to buy your Guaranies... but I think a trip to Agustinas would be more exciting (:
This part of Santiago is not exactly dangerous, but it's not the safest part, either. Pickpocketing and snatch-and-run crimes are fairly common in this part of town, so when you come out here, you might want to leave your valuables at home, and don't bring HUGE amounts of cash with you.
Agustinas is about 2 block away from a Metro station, or you can hail a cab in the street if you don't want to walk a long distance after walking out of a currency exchange.