One of the quieter encores I've played is the Ronde française, by Léon Boëllmann. Scroll down this page (selection #5) to hear a sample from an album by John Longhurst. It is relatively easy (pedal is optional) and ends with a quiet glissando - this sometimes elicits a surprised 'ah' from the audience. 
The Boléro de concert by Lefébure-Wély is a fun crowd-pleaser. I've also done Vierne's Carillon de Westminster.
Another nice piece is A Trumpet Minuet, by Alfred Hollins (though maybe it works best within the recital, rather than an encore
).
Dupré's Placare Christe Servulis, from "Le Tombeau de Titelouze" is excellent indeed. I also love Dupré's Magnificat VI: Gloria (Finale) from the fifteen pieces founded on antiphons - it is quite short, and loud! For a sample of Judith Hancock playing at St. Thomas, see this page and scroll down to selection #15. This sample includes nearly the entire piece.
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