Unfortunately the answer is no!
All Toll exits on Portuguese Toll Motorways always have at least one toll booth manned, with a green arrow indicating it.
Many people have been in the situation that you have described and have pass through the electronic Via Verde toll booths. The toll operators show no mercy, on the basis that if you are driving in a country you are not familiar with, it is up to the driver to familiarise themselves with the rules of the road in that country before driving.
