I will be returning to Lisbon after my Camino as well. I explored one way car rentals, and if money's no object, you can do it. For the dates I wanted, the quotes were over $1000.
For me, the only viable option is the bus. Trains require two changes, one in Vigo and one in Porto (of course if you're going to do some touring on the way down, that might work fine for you). The bus is direct, but is long and a very dull ride -- I've done it three or four times already. The bus leaves Santiago in the morning around 10 or 11 am and arrives in Lisbon around 7 pm. Spain is an hour ahead of Lisbon, so that adds an hour, too. Depending on the passenger list that day, you may stop at a lot of cities in between or very few.
If you find some other great alternative, let me know, because I do not enjoy that bus ride! Laurie
They do allow it, but it is expensive since you are leaving it in another country.
The alternative would be to take the bus down, or the train (http://www.renfe.es). I would recommend the bus, it is faster. Also I think that there is a flight from La Coruña to Lisbon, might be worth a try.
The problem is the crossing of the border. You'll be starting with a Spanish car and returning it to a Portuguese base. No problem if you stay within the country - I've done it several times when arriving and leaving from different airports on non-camino holidays in France - but as soon as I wanted to return a French car to Spain ......wow !