I have not opened my DSi up yet but I doubt what I intend to do is doing to take up much room so I should be in the clear.
I intend to take my DSi apart and drill a third hole in the lid, across from the purple led. I am going to put a push button flash there. It'll just be a really bright white led but it'll serve it's purpose. I have not decided on where I am going to put the button, however I do have the button sitting here, it matches the start and select buttons on the DSi. I might try to get a power button instead and put the button for the flash right above the power button.
I want it to look as original as possible, and although a push button led is in no way an original mod is has nothing to do with originality nor whom did it first. I need functionality and a flash on my DSi so if I accomplish that, that is all that matters.
-TwisT