100% open space regulation

I have never had this particular problem, but I think there may be an important distinction here between the LEED requirement and your code, based on your description. The credit intent is to reduce the development footprint (building, hardscape, roads, parking) so to increase the amount of vegetated open space. It seems that your code requirement doesn't necessarily limit the amount of development, but simply requires all left over space to be vegetated. I think given that distinction, you could argue that you fall into Case 3: Sites with Zoning Ordinances by No Open Space Requirements in which case your vegetated open space must be equal to or greater than 20% of the total site area.

I'm inclined to agree with Ellen. The zoning is saying that no matter how little open space is remaining, plant something on it. So you could literally pave paradise and put up a parking lot and be zoning compliant. Case 3 should guide you to making more sustainable choices. You may want to add a narrative to the credit submission.