Solved! In the end both Alvar and the above mentioned question pointed at the right direction. The solution is really meant to be "layout changing" in Ubuntu... even though I still strongly disagree with such a label: the change is in behavior of certain key combinations (input) to achieve desired characters (output), not in layout (physical arrangement). So, as Ubuntu wishes, changing the layout to English (UK, international with dead keys) solves the problem... and creates another: now I need to press " + space to get " ; at least now I can easily get á, ç, ã, ê, ü, etc.
Uma citação direta do comentário sobre o qual Rodrigo de Oliveira postou sua solução.