Found it! Searching the Hillier Manual of Trees and Shrubs on Google books (much quicker than searching my own copy!) I found a reference to Carl Miller and Sir Harold Hillier (himself), (M. & H. 308) , having collected seed of Corylus sieboldiana in Korea in 1976!
However, the entry under Paulownia doesn't have any mention of coreana (only P. tomentosa "Lilacina" syn P. fargesii)
However, Hillier did introduce Carpinus coreana on the same trip.
Harold Hillier relinquished his duties becoming President of Hilliers Nurseries in the mid-70s which allowed him to travel. Apart from the Far East he was also in "the Americas", so perhaps he even took the seed with him to the US, inspiring the new trip later in 1977?