A suggestion if your enquiry draws a blank.
Essentially, design and make a half-nut held on a die-block working in a slot in the reversing-lever, to accomodate the vertical displacement due to the lever's arc. The nut would be operated from the trigger that would otherwise operate the quadrant catch; the thread itself giving the notches on a "quadrant" of infinite radius. (c.f. the toothed rack)
Provided the engagement is sufficiently positive, you should need only the upper half of the nut, otherwise you are looking at a clasp-nut worked by a cam or double-ended lever, similar to that on a lathe.
The thread would probably have to be of square form to remove any ramp-climbing tendency, and the lever preferably double-sided over the screw to support the die-block from both sides.
Have you tried contacting the RH&DR for information?
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I believe there was an early 19C standard-gauge design for such a combined control, but which overcame the arc displacement problem by using a barrel-shaped screw. I think it was found too difficult to make economically; but I imagine it would have required either a thread-milling capacity very advanced for its time, or much simpler and much more likely, a centre-lathe fitted with a profile-follower.
I do not know how it handled the question of thread-flanks not radial to the reverser-lever fulcrum / screw-radius centre. A profiled thread perhaps; the half-nut or catch made to swing about its own centre, or the nut "thread" being horizontal pins like a segment of a lantern-gear.