I found
@Rodrigo Cerqueira’s thread about the boardwalk/riverwalk that might change some of the route out of Lisbon. I don’t know what the status is, but that could make a big difference in the early stages out of Lisbon. It cuts 7 km off the route and avoids Alpriate. But as I said, I have no updated news on the possibility that it will happen.
My observation now, after 5 years part-time in small-town middle Portugal, is that once a trend is set, everyone else piles on. I think this is very much the way with "riverwalks."
Ribatejo and Mediatejo smaller-town districts (Central Portugal, along the Tagus) have observed the success of the tourist industry in the Algarve, and more recently in Lisbon and Porto. They acknowledge that they don't have as many high-profile tourist attractions (beach, beautiful old cities, urban "buzz") and are casting around for ways they can make themselves more attractive to international tourists.
Nature walks, and particularly walks along the rivers, have become a good option for the smaller towns that have nice river edges. The national government has been supporting their initiatives with a good percentage of the funding for creating these. As well, there is EU money for bootstrapping the southern European countries (Portugal, Spain, Italy, Greece) to pull their incomes up to the EU standard, so funding comes from that source as well.
Our municipality (Vila Nova da Barquinha) has just opened a 10+ km walk along the Tejo from one end of the municipality--VNdB on the west--to the other--Praia da Ribatejo on the east. The opening-day hike along the route happened just a few weeks ago, and everybody locally is quite excited about it. \
More recently, I read a story in one of the local Portuguese news sources that another municipality, about 50 km east of us, has just done the same thing. I think we're going to see more and more of these. Eventually, I suspect there will be a riverside walk, all the way from Lisbon to the "turn of the Tejo," here in Barquinha, and easterly beyond, though that will be less interesting to caminho walkers who are heading north.
Mind you, it could mean that walkers would be able to combine the southern part of the Nascente caminho with the "north of the Tejo" section of the Central from Barquinha north, linking the two with a walk west along the Tejo. That would make an interesting route!