Fr Tom, there are a few members who have travelled a route backwards successfully, so take heart that it is possible. If you are relying on the waymarks, in most places this works well going forward, but when you do find one when going the other way, it might not always be obvious where they were expecting someone to come from to get to it.
You might want to use a GPS app that will do that. Most general purpose GPS apps like OSMAnd can reverse a track for you to follow, or you could use something like the
Wise Pilgrim app and follow the track backwards.
Once you get past Porto, it will be possible to follow the waymarking of the Fatima routes for a while, but if you want to follow the Camino Portuguese route, you will need to take care where different Fatima routes head off. Once you get south of Cortico, I don't recall there being another point before Tomar where you would need to be careful.
There are then Fatima routes from Tomar and then Santarem, when you would be following both routes backwards,
Good luck.