you looked at the diana underlever/sidelever platform Nick ?

Some interesting attributes that are of value when looking at long strokes..

Long cylinder/loads of spring room
Long (100mm min) strokes as std
28mm easy to find tube to sleave down
short stock TP arrangement
the one piece design is no issue for o-rings, as you are sealing 1mm shy of the front of the tube anyways
spring loaded sealing on the underlevers