Called the 1-800 Brazilian Customer Service Line and was able to secure seats for my award flight issued by Smiles (Gol Linhas Aereas mileage program).

Manage my Booking options (website and mobile app) still show the "cannot make changes" warning though.

Not sure if I got lucky or Avianca is changing its policy on selecting seats thru the phone.

Made a dummy booking for the same dates and when I got to the seat selection screen saw that the agent secured my seats indeed.