One of my Pipe organ parts supply company's Catalogs, from 10 years ago, shows a two part Trem control device, ie;
(1) a small "pc" board, with both a speed pot and a depth pot to mount on the backboard, also knobs for them !
(2) a wooden box, with an air connection to be tubed to the Pipe-chest -- this box had a large Electromagnetic Air Dump Valve inside it that would be
"Pulse, driven, with console adjusted "Pulse, speed" and length controlled by the pots on the Console Backboard . I would imagine that a swell slide switch could be connected to the speed pot to get the desired effect (On a Pipe Organ, -ONLY- ),! ! !
-----On a Hauptwerk Organ it would be difficult to Impossible to implement,
unless provided for,in a sample set ! ! !
Best wishes to All !