It's very uncommon to find a monitor or TV that actually does non filtered integer scaling, most apply a blurry bilinear scale that looks awful. And afaik there are no external HDMI pixel doublers on the market, I would buy one in a heartbeat.
Native 720p does look slightly better than integer scaling to 2x because of subpixel stuff, and generally liking that LCD look. But integer scaling would be enough really.
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?
Native 720p does look slightly better than integer scaling to 2x because of subpixel stuff, and generally liking that LCD look. But integer scaling would be enough really.