I will try Flyfishing for the Zeds as well, this spring. A friend of mine does with good success on the river Rhine (harbours).
As i have fished the Ebro reservoirs myself three times i know what you are facing and i can tell you how i would approach them with a fly.
If i had a sinking line, i would use it (you know how deep the water is ...). If not, i would concentrate on the smaller bays and fish with a bunny hopper - style fly (i have tied some really neat ones with two carp hooks and a soft braid line inbetween, so they really behave very "wavey"), maybe putting a lead shot onto the leader, to get down to depth (I use a sliding lead between 0.5 and 1gr in front of the streamer, stopped with afloat stopper 10 inches in front of it, that way it plays much better)
I would also use tube flies. I guess, any Streamer will do.
Colours ... Well, it depends, as always. I´d start with the natural range in white/glittery and change it to see, what they favour.
If you get a catfish on the fly, cut the line
Maybe start to fish with a spinning rod and lure to find the fish (if you haven´t got any sonar) and then change to the fly rod ?!
Yes, thats what I am planning on doing. There are three of us and we are mainly lure fishing but if a lot of fish start showing up then I'll pull out the flyrod. Thanks for the tips. Bunny hoppers eh... hmm.... on a 6inch wire trace...