I walked the Portugués Central in January last year, and because the days were short and so was the time available to complete the trip, I wound up walking in the dark for a decent period almost every day: before dawn, after dark, or (usually) both. I've also done quite a lot of such walking on parts of other routes (Francés, Primitivo, etc.) when I've a long sector to cover. Here are my thoughts:-
1. It's manageable, but I wouldn't like it to be my first experience of hill-walking over unimproved surfaces at night. Practice doing so is necessary. Lots of practice.
2. Nothing special is needed in the way of light sources—those little hand-carried LED things, which weigh only a few ounces—were perfectly adequate for me—but you do need several of them to be absolutely sure that you'll always have one as a back-up. Otherwise, if you're stranded out there without light, you're immobilised until such time as the rosy-fingered dawn puts in an appearance.
3. I didn't notice that the dogs were more dangerous and/or aggressive after dark. (They're a bloody menace at any time.) One might notice them more because they detect one's presence, and start barking, from a lot further away.
4. It's a slower way of travelling, because navigation becomes a greater difficulty. No matter how good one's eyesight may be, one misses a lot of arrows in the dark: for me, around half of them. This is because so many are painted in odd places that swim into one's field of vision easily enough during the daytime, but that one won't see at night unless one is specifically searching for them. Lots of arrows are also so faded that, while more or less discernible in daytime, they're virtually invisible under artificial light.
5. The degree of difficulty in this respect increases greatly during precipitation and, especially, mist or fog. The latter are much more common in the night-time and pre-dawn hours than in the day for basic meterological reasons: when the sun is down, the temperature sinks toward the dew-point, making mist and fog formation possible. On a very well-marked and unobstructed part of the trail, one might manage to make slow progress even during misty conditions. Otherwise I'd stick a fork in it those nights; hole up; and wait for better weather.
6. Either way, though, the basic rule of night-walking is never to leave a known position unless one is absolutely sure that one is heading in the right direction. (Having a light and cheap magnetic compass, incidentally, is immensely helpful in confirming that one is not heading off in the wrong direction entirely.) This meant that at every fork or potential turn-off along the trail I had to stop and search the area with my torch to find the relevant arrow. Sometimes, when I couldn't find it, I was obliged to retrace my steps to the previous one and try again. Factor into your daily distance-calculation the necessity of doing this.
7. The same is true when negotiating difficult terrain. It's possible at night; it just takes longer. On a slippery downhill stretch over rocks, one has to illuminate and consciously choose with each step where to put down one's foot. Not a problem if one is disciplined and cautious, but it all takes more time. On a daytime hike along these trails, I average 5 km/h pretty consistently; six if I'm pushing it (e.g. to avoid losing the bed race). At night, 4 km/h is an excellent rate of progress for me, and there's no such thing as pushing it. Not unless one is looking for a broken or sprained ankle, or a bruised coccyx.
8. You do forgo the possibility of observing the nice parts of the countryside, and of visiting churches and so on (although, God knows, few enough of those are open in Iberia in the daytime either—one of my major pet peeves as somebody who does these things for religious purposes alone). At night, though, the "scenery" is the sky, and I've seen some marvellous things there while on pilgrimage, especially out in the country districts where there's no light pollution from towns or street-lamps. Quite a few of my very nicest pilgrimage memories are of the hours after dark.
With preparation and an appeciation of the inevitable compromises and trade-offs one is making, then, it's perfectly possible. A different way of doing things, to be sure, but there again, as is a common saying on this site, every person's Camino is different.
Best of luck to you both.