This sounds like a workable solution for the rock bottom ponds. It would
work much more like the undergravel filters of aquariums since you would be
pulling water through and then filtering, etc. One caution would be to put
sufficient support under the grates to be able to walk on the rocks.You
never know when you will have to get in to rearrange pots, catch fish, etc.

Yes, you really need to use some kind of support pallet for this.

I'm not really trying to turn it into a filter; just a single layer of
hand sized flat-round stones. There will be sufficient gaps to allow
certain kinds of wild life to live, hide under the "false bottom".

I don't think detritus will accumulate down there; a mild pull will
exist from all times from the bottom drain...

C//