Yes you can transfer on VY flights at BCN.
I was thinking SAS might have been a better punt than DY because of the better sized aircraft and onward connections, yet DY does offer much of the same with lower fares. Could we even see DY open a base at NCL with 1 738 based if they get their Irish AOC? That would be nice to see, they could open up untapped markets such as Morocco and some Greek/Croatian markets. Then the obligatory PMI/TFS runs and maybe some Italian and Spanish cities?
I think if Wizzair have made such a good go of DSA, NCL should work pretty well for them. They're bloody cheap too! The only airline I know of that was as consistently cheap as them was Bmibaby. Ryanair and Easyjet occasionally but not as consistently.
El Al and Turkish would be really nice to see.