Hi,
You could try storing the texture as non power of two, not putting it in a sprite collection, and use tk2dSprite.CreateFromTexture(...) to create it. I'm not sure if older iPhones support non-po2 textures though - you'll have to check that. If you're doing that, you'll have to manage resolution switching manually (using resources.load) but that shouldn't be too bad.
As far as I know, 3GS does 2048x2048 textures, so you could do that to support iPhone5.
For full screen textures, explicitly loading textures is probably the easiest way to handle this. You can then load specific images for whatever resolution you're dealing with.
Sprite dicing could help, but you'd need empty space in your textures for it to be of any help. It will massively help if your images aren't fully solid.
I suggest using tk2dCamera so you automatically get everything scaling properly - even on android. You can then use overrides to manage different related (scaled) resolutions if you need to.