I've been continually referencing your forum while building my greenhouse, but I registered for this thread
A couple of things spring to mind. First, start with reform and conservative and modern Orthodox congregations. They might bring up Halacha concerns, but they aren't going to be sticklers for what's in the Mishna - the spirit will matter more than the letter.
Regarding the prohibition itself, I (not a Torah scholar) would be FAR more inclined to think it somehow related to the leaving the corners of the field unharvested. I think if you can show that permaculture is about first returning an unnatural habitat to a more natural state and THEN planting your 'crop' seeds, and also if you stick to rows and don't mix the seeds during planting, you can make a case that part of this was perhaps to make it easier for the poor of a community to know when a field is open to them. If there are multiple crops, how does someone walking by know whether the field has been harvested or not? How do they know what they can take and what they must leave? Multiple crops makes this much more difficult, and I think 9/10 rabbis would agree leaving the corners for the poor is the more important mitzvot vs. in what order you return an ecology to a plantable state.
If you go in with a solid plan for observing the loosest interpretation of the sowing law (rows, don't sow the seeds at the same time), and ALSO a solid plan for observing a stricter interpretation of the corners of the field law (can you get the bar mitzvah class to harvest and take it to a food bank or an organization in a food-deprived neighborhood or the local Jewish nursing home? Or ALL THREE?) I think you'll meet open minds.
