Wren's design for his St Paul's Cathedral organ cases originally incorporated vertically sliding sash windows over the pipe fronts that protected the organ when not in use. Some of these survive in the works department, according to Plumley and Niland's history, which illustrates them. A similar arrangement was seen in the 17th century consort organ known as King James's Travelling Organ, sold to a buyer in the USA in the 1920s (by which time the sashes were missing) and now of uncertain whereabouts. The organ with the sashes intact is illustrated on p.275 of Boeringer's Organa Brittanica vol III if you have it, or if you search for "Stowe catalogue 1848" on the internet you should come across a facsimile of the sale catalogue from which it is taken - it's facing p.245.
Hinged glazed doors became a feature of many 18th century chamber organs, but is anyone aware of any other examples of sliding sashes on organs, especially from the 17th century?
PS Another example appears to have been found on the organ at St James, Piccadilly: originally built by Harris for the Catholic Chapel at Whitehall, it was installed in Piccadilly by Smith in 1691. According to Barbara Owen, the sashes were kept closed when the organ was used during penitential seasons. Is a Smith peculiarity emerging here? Could this be an alternative source of inspiration for the first swell boxes?