Wow BleedingStar, I wonder what kind of building that might be that your room would have 20 foot ceilings??? You are absolutely right that you should get your PJ in there and try it--you'll have a great advantage with all that wall space to shoot at.
The divider option is the only one *I'd* consider--apart from the distance & difficulty of running cable to your ceiling, then dropping-down 10 feet again (at least), you'd need a very serious pipe to assure the pj won't wobble at all in operation (internal fan), or from a/c airflow maybe. In any case for testing you'll want to use a shelving unit to project onto the wall, and one that you can move around, particularly back-and-forward.
Couple thoughts:
1. You gotta think hard about your room usage and possible placement of other desired features e.g. wet bar, pool table, tennis court
, etc. unless truly you intend only to dedicate the room as a theater space, and then
2. Consider all the AUDIO issues you'll have. It's a cavernous space and acoustics are going to be awful unless you partition it-off somehow, or plan some pretty major ceiling (and wall) baffles/curtains/carpets to manage the sound.
You're very lucky as you'll have lots of options--good luck with whatever you decide to do!