One for Mr M.

Erm, don't quite understand the question-
you want to bring the gearchange up through the seat, so it's in your lap, as my last trike was? That's the ideal position for keeping gearchange sloppiness to a minimum, but you may have to sit a little further back than you planned -don't forget you need to be able to change gear when wearing bulky waterproofs for example. I screwed a short extension onto the original gearstick to bring it up above the seat padding. However, by extending the stick, I increased it's travel between gears -obviously you don't want it hitting a frame tube before it's engaged the gear. Again, don't forget you need to try that with heavy gloves on as you might find your gloved fist hits the back of the tank when you change into first & third for example.
Can't really advise much, other than to try it in all the gears & see if it fouls anywhere. My own current trike build features a gearstick with a couple of bends in it that exits sideways, just under my left leg. Very comfortable to use, but also very sloppy with lots of travel between the gears, so I might rethink it before it's finished.