This is a quick drawing I made in SketchUp of an idea I have. I may be so far in the weeds, I am out of left field entirely, but here is a solution I hope will work.
Essentially, this is a hardwood frame about 53" x 21", from 3" x 3" ash. Toward the top is a ratched and pawl with a rope takeup beam (not sure what to call it), with the actual round ratchet and pawl mechanism not really show here, but indictated slightly the the little yellow ring at the end of the takeup beam. Not show is a strong pully, which wiill be attached to the top horizontal part of the frame, on the inside. All joints will be half-lap or saddle joints, reenforced with bolts.
Toward the bottom of this contraption is two bars. They will be padded with leather, and you would slip the crossbow in there. It would be something like that illustration in Ralph Payne-Gallwey of the early crossbowman (Fig 24) spanning his weapon by his feet on the prod next to the binding. On the other side is a bracket that you could use to span the prod not mounted on the tiller, similar to a tillering tree used by hand bow makers.
The series of 1" holes drilled up the two side pieces is an idea I took from the Asiatic composite bow world. High powered composite bows can be spanned in this way, by slowly bringing each arm of the bow down and slipping big pegs onto a gigantic flat board to hold the limb tips until you get to full draw and then string the bow. I'd use 1" hardwood pegs, and once the ractchet and pawl has done it's job and the prod is sufficiently spanned to allow the string to be taken or off, slip in a peg and do the job.
Any comments are welcome.