Hi Jim,
I think the big advantage of your design is that it doesn't necessarily have to involve the use of Oz clips. Many people don't like the additional holes in the frame and they are kind of pricy anyway. I have seen them fail when used improperly as well. As you noted you would have to add material to attach them to since that typically isn't possible in the corners. That is probably why the traditional variation just involves two movable planks that run vertically inside the travel frame that the Oz clips (or before that "L" or "T" brackets) could be attached to. That way the piece could be centered (if the painting is way smaller than the frame then the whole thing won't be off balance when you dolly it etc...). Oz clips also get dicey when pieces get big because they start needing to be attached to the other two legs of the frame (especially the top as their weakness becomes apparent during a topple event).
All that mess is why the corners you are using are very appealing it seems to me. If those are sound then you are really mostly looking for a way to secure the painting back on to them. There are several kinds of webbing that along with the use padding for the frame surfaces that might be worth considering.
Good stuff! When you end up making the real thing be sure and send in a picture.

Ashley