This is what I would call ASB territory. The British royals stuck to marrying other Protestant Royals and to a lesser extent British nobles for most of the 20th century. By the 1970s it became commonplace to allow the British royals to marry for love. This allowed commoners into the royal family. This was also the case in most other European royals. So unless a British royal falls in love with a member of the Japanese imperial family after the 1970s, I don't see such a match taking place.
Below are the marriages of the descendants of George V. Most were to British titled aristocrats and later in the 20th century to British commoners. Only a few were to foreigners.
1922 - Princess Mary marries Henry Charles George, Viscount Lascelles, elder son of the 5th Earl of Harewood.
1923 - Prince Albert, Duke of York marries Elizabeth Bowes-Lyon, granddaughter of the 14th Earl of Strathmore and Kinghorne.
1934 - Prince George, Duke of Kent marries Princess Marina of Greece and Denmark.
1935 - Prince Henry, Duke of Gloucester marries Lady Alice, daughter of the 7th Duke of Buccleuch and Queensberry.
1937 - Edward, Duke of Windsor marries American divorcee Wallis Simpson. He had been forced to abdicate the throne in 1936, rather than marry for love.
1949 - George Lascelles, 7th Earl of Harewood married Marion Stein, a commoner of Austro-Jewish ancestry.
1952 - Gerald David Lascelles married Angela Dowding, daughter of a commoner.
1960 - Princess Margaret married a commoner, Anthony Armstrong-Jones, the Queen giving him the title of Earl of Snowdon.
1961 - Prince Edward of Kent marries Katharine Worsley, daughter of Sir William Arthrington Worsley, 4th Baronet.
1963 - Princess Alexandra of Kent marries Sir Angus James Bruce Ogilvy, son of the 12th Earl of Airlie.
1972 - Prince Richard, Duke of Gloucester married Birgitte van Deurs, daughter of a Danish commoner.
1973 - Princess Anne marries Captain Mark Philips, a commoner.
1978 - Prince Michael of Kent marries Baroness Marie-Christine von Reibnitz, daugther of Baron Gunther Hubertus von Reibnitz. Becaue the Baroness is Catholic, this marriage leads to Prince Michael's forfeiture to his rights to the British throne due to the Act of Settlement in 1701. Their children however, retain the rights since they were brought up in the Church of England.
1981 - Prince Charles, Prince of Wales married Lady Diana Spencer, daughter of the 8th Earl of Spencer.
1986 - Prince Andrew, Duke of York married Sarah Ferguson, a commoner.
1988 - James Ogilvy married Julia Rawlinson, a commoner.
1988 - George Windsor, Earl of St. Andrews married Catholic divorcée Sylvana Tomaselli. Their three children were raised Catholic, removing them from the lines of succession.
1990 - Marina Ogilvy married Paul Mowatt, a commoner.
1992 - Lady Helen Windsor married Timothy Verner Taylor, a commoner.
1992 - Princess Anne, Princess Royal married Sir Timothy Lawrence, a commoner.
1993 - David Armstrong-Jones, Viscount Linley marries Serena Stanhope, a commoner.
1994 - Lady Sarah Armstrong-Jones marries Daniel Chatto, a commoner.
1999 - Prince Edward, Earl of Wessex, married Sophie-Rhys-Jones a commoner.
2005 - Prince Charles, Prince of Wales, marries Camilla Parker Bowles, a commoner.
2006 - Lord Nicholas Windsor married Paola Doimi de Lupis de Frankopan, daugther of Louis, Prince de Frankopan, Count Doimi de Lupis. Lord Nicholas converted to Roman Catholicism in 2001, forfeiting his rights to the British throne. In addition, his wedding took place in the Vatican.
2009 - Lord Frederick Windsor marries Sophie Winkelman, a commoner.
2011 - Prince William, Duke of Cambridge marries Catherine Middleton, a commoner.
2011 - Zara Philips marries Mike Tindall, a commoner.
2012 - Peter Philips marries Autumn Kelly, a Canadian-born commoner.