Blast! Apparently even I failed at simplicity!
Lunits, I intended my clarification to be even less restricting than yours, only restricting the space "below the HIGHEST point of that tire". So take your solution and make X=zero and we are saying the same thing. It also would leave most undertrays untouched as it only restricts the space "directly above the top surface of a tire ". I think your solution is reasonable but I would prefer an even less restricting rule.
I also think removing the open wheel requirement altogether would be reasonable (and awesome, for the same reasons mentioned by Kevin).
Doug, I thought about a clarification with similar wording, but went away from it because I feel it would require definitions of bodywork and aero devices (easier to ban everything), and it would effect the space inboard of the tire. I could imagine people (me) making suspension links with intended aero effects.