Hi again,
I'll buy the local newspaper tomorrow & will take photos of the ads.
The more I think about it the more I realize you'll have trouble finding such a place to live in.
They do exist though but I doubt they will be of a standard that would fit your taste mainly because such short term rentals here target lower classes of people.
I may have a solution though (with pros & cons) :
My neighbour is renting such a room in her house, with access to the kitchen, shower & such. She's about 60 years old, works in Capital (sleeps there) from monday-thursday. She's living with her son (20 years old).
Pros : room (quite nice, was just renovated) in a nice house with a really huge park (4 hectareas = about 36.000 sq ft), in a gated community (safe). The park itself quite be quite enjoyable since spring is here (there's also a pool but they don't clean it only twice in the year so mostly it can't be used). They have internet also so you'd have the possibility to keep connected.
Cons : that's on "Ruta 8" (nearby the intersection with "Ruta 26" for your Google map search) and San Miguel is like 20/25 minutes away by bus (at least it's direct).
At night, there's nothing much to do in the area = a disco on weekends which is not too bad (1980s music), a "bingo" where there are Texas Hold'em cash games & tournaments, restaurants suck all around except the Sheraton one in Pilar (pricey), etc.
Other than that, your best bet would be to go to San Miguel early in the morning & spend a day there trying to find a place (visiting each & every real estate agent, looking at windows on kioscos/panaderias for such ads, buying the local newspaper & making phone calls, etc.).