Okay, here's the issue. The DS has no operating system, so you cannot run more than one program at once. Once the game you want is loaded, the "video capturing" homebrew would be shut out of memory entirely and the game would take over. Thus, the only possible option would be to patch the rom directly, but then the question is, where would you store the captured images? And then there's the fact that the DS hardware only has a function to capture a screenshot of the primary screen (which is chosen by the developer).
So in essence, even asking for in-game screenshots is a terribly high request. I believe the CycloDS Evo team was looking into supporting in-game screenshots, but only a few at a time due to memory restrictions, and you'd have to store them in RAW format until you had the chance to convert to a human-viewable format. I don't know if anything ever became of that.