SD to USB adapters exist, but is it possible to hook up an SD-to-USB cable and have a driver present a virtual sd disk to the camera but write the video directly to your computer RAM via USB?
That would solve the problem for all dSLRs.
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
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 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.