Has anyone ever designed a SW transmission system using
curtain arrays that has a beamwidth of 2.5 to 5 degrees?

I'd guess that if it were done, it would've been inspired by the
cold-war broadcaster vs jammer "contests".

If you can find pictures of antenna farms from the 50's/60's/70's
(sometimes pictured on QSL cards for example) you might look for
patterns indicative of the arrays you're interested in.

Tim.