If you mean the ceilings slope up like a dormer from two sides (like the two sides of an A) to a high center peak, you could find what ideal height you want the lights to be at (10' ?) and attach a 2x4 or 2x6 along one wall, run a level across to the opposite wall, then using joist hangars, run perfectly level joists between them, like you would if you were installing a level ceiling. You'd only need cross-joists wherever you plan to attach two ends of each light, and perhaps wherever you might plan to install a soft reflective pseudo-ceiling (like stapled Orca film).
That way, you have a level area to hang your light pulleys from, so you could raise and lower the lights at a continuous horizontal level to however close you wish to be to the crop canopy, raising it as they grow taller.
When you attach the first two boards to the two sides of the ceiling, use a stud finder to locate the existing ceiling joists, then use 4" screws to attach to them. That way, they'll be secure and at any point you wish to disassemble your new ceiling, you can patch the screw holes in the old existing sheetrock pretty easily, touch up the paint and it'll be back to normal.
If you do this, you still may get some heat rising above your new pseudo-ceiling, requiring some added venting, but maybe not. Once the lights are hung, use a thermometer up near the existing peak to determine that.
Of course, if I'm envisioning the shape of your existing high ceiling wrong, then forget everything I just said!
