I did look into it a little bit, I think the reason none exist is because it's actually pretty difficult.

Taking in a 720p hdmi signal and pixel doubling to 1440p or tripling to 4k and outputting that as a valid HDMI signal without adding more than a frame or two of lag would require custom chips. Although maybe a software solution involving a gpu might be possible?

FPGA is probably a safer bet if you want to minimize latency.

Implementation is left an as exercise for the reader.

Take a look at the HDFury products. The X4 should do what you need with those lower resolutions.

Interesting. Did you investigate FPGAs?