Particularly for an 'experimental' event I wouldn't try to second-guess things very much. Rather than fixing motor, prop, etc., just dictate line lengths and a maximum battery weight (I suggest 1.5 or 2 ounces for a "15-sized" race), and that at least one battery change and restart per race must be done. Let people mess around with cell counts, props, motors, airframes, etc., to their hearts content. Since ejected batteries are a concern, maybe make a rule that they have to mount from inboard, and make any sort of a "dangling battery" grounds for getting no score for that heat.
Then stand back and find out what solutions people find. I don't think things will get too wacky, but if someone shows up with a 200mph racer that needs five pit stops per heat you can always impose a speed limit, or put a maximum on battery changes.