I think we came up with a workaround. What we'd do is crop the 1366x768 width-wise to 1024x768 for everything, and use that for all iPads. Going retina it too heavy for mobile devices. Next up, we'd have two sets of assets for backgrounds: 1136x640 for iPhone and 1024x768 for iPad. All other assets that would fit in the room could be drawn at iPad-resolution, and scaled-down programmatically for iPhone. Scaling down with linear filtering seems to look nice, and although we're still adding many more pixels to the screen for these smaller screens, we don't have to double everything which allows us to meet managements goal: keep it under 50MB.
EDIT: I'd like to mention that the iPhone 5's 1136-width for native resolution is obviously too large to fit into a texture, so we crop the width down to 1024 on each side. That way, the main part of the texture takes up 1024x640 of the 1024x1024 texture. The sides that were cropped get added in the unused region of the 1024x1024. They have to be rotated to fit properly.