My DSLR camera has a mini HDMI out that you can use out of the box, doesn't require the camera to be in recording mode or such. It simply gives you the live video feed -- rather than having it write to SD card, having software on your computer decode the video & show the last frame there.
The only option I think (it's been a while) I had to vaguely enable is to have it not show the normal camera UI over HDMI, but that was a simple setting.
Having hardware emulate a SD card sounds like a niche product, if anything, whereas the only item I needed for my camera was a mini-HDMI to HDMI cable.
The elgato is doing something completely straightforward — video output to video input, done. With only a few details, it’s almost like a regular webcam where the controller is in the usb plug rather than the webcam’s body.
The SD thing has all sorts of nastiness to it.
First, you won’t have a standard usb video input, so you’re going to need to do a bunch of awkward driver work to get applications to consume the video feed.
Second, you’ll have to deal with the lag and jittery video that you get because the camera will almost certainly buffer writes.
Then you have to figure out how to get the controller in the SD reader to understand the incoming writes, which it was most certainly not designed to do.
edit: if the elgato can do it with HDMI, the question remains can it be done directly with SD out?
e2: also, SD card readers/adapters are cheap and plentiful, this seems like it should be easier to implement