This link may be of help. http://baexpats.org/expat-life/11947-how-go-el-palomar-airport.html#post85696 My personal view is that the LSM is okay to use at any time though I accept that others' mileage may vary on that matter. I'm getting increasingly wary about entering and leaving the actual Retiro station late at night though the inside of the station itself seems safe enough. If you are coming and going from Palermo station there shouldn't be any issues.
ETA: The fare from Retiro to Devoto is one peso so I guess Palermo to El Palomar will be less than two.