Wondering if i'm not in over my head with this, but i'm trying to trick my 3DS into playing videos off of my computer without having to buy an R4 card (which may very well be illegal in Japan)

The end goal here is watching episodes of Fairy Tail on what is inevitably going to be hour-long daily commutes to and from campus. Having put my 3DS SD card in my laptop, i discovered the location for where the video you can record from 3DS goes to, and tried ripping other .avi files into there. That did not succeed, so i tried renaming the file into something matching the other files in the directory, which also did not succeed

I'm wondering if i have to do some editing of the .avi files on my side, getting them down to the same resolution (an odd 480 x 240) seems the best start, or if this is all a fool's errand because there's something more insidious in there, or just some sort of file tag, which would throw this all into "more trouble than it's worth" territory...

Try getting it to 400x240. You never know! It might work.