I know IB did the MAN run from Madrid with the Air Nostrum CRJ's as a short lived stand alone service previously..... but back then they were up again 2x daily BA CitiExpress or whatever guise they were in at the time. If my memory serves my correctly I think thats around the time ZB threw their hat into the ring too with their daily A320 too meaning there were far too many seats available to sustain everyone.
Difference is now we have nothing and are crying out for this service to be reintroduced by somebody. I personally think 2x daily Air Nostrum CRJ's would be a good starting point and an instant hit. I would much prefer this option over somebody like WW having a go. With the right timings IB / Air Nostrum could feed into the IB South American network and also cater for the point to point mix of business and city-break traffic.