It's allowed. You just need to walk the last 100 km on a recognized route and so long as you cut over before entering Spain and walk the last 100 km in Spain on a recognized route (which would include the Coastal or the Central, with our without the Spiritual Variant) you are good for a Compostela. If you aren't worried about obtaining a Compostela, everything is allowed!
I walked the senda litoral out of Porto along the river and up to Vila do Conde before heading over to the Central route. That gave me a nice walk out of Porto and a taste of the coast, as well as Barcelos and
Casa da Fernanda and the other good stuff on the Central. Another option that I've heard of that I might try on a future Camino would be to start as I did: senda litoral out of Porto and then over to the Central from Vila do Conde, but then when I got to Valença heading back to the coast at Caminha and walking the Coastal in Spain where there are some nice dramatic coastlines. That gives you more like half and half between the Central and Coastal. I would miss out on Tui (unless I crossed the bridge for a visit and then crossed back again) and O Porriño, but I would get to see Caminha, A Guardia, Mougas, Baiona, and Vigo.