What would be the point in doing that? You would be going off track by 1,000 miles just to go through immigration. It doesn't seem worth it to me.
It only works from Dublin because you are going towards America. Unfortunately I can't see too many people flying to Madrid for preclearnace.
It's great to get preclearance but its not the end of the world if you don't get it, i'm not going to detour for hours just to get it.
No point, clearly.
People won’t fly from the UK to MAD for pre-clearance, but it may attract pax from southern Europe to fly on IB, just like it is expected that some pax from northern Europe may use EI with pre-clearance at DUB.
Willie Walsh is the head honcho of IAG and that includes IB as well as BA.