I think the smallest you're gonna see for a 5gal keg is a home built one using either a mini fridge, or a small chest freezer with an external control. I built one using a 5 cu ft chest freezer, external control from Johnson controls and tap in the top.
As for on-demand cooling, there are lots of cold plates out there for this, think picnic cooler filled with ice and a tap on it. Don't think they'll work unless they're in ice. Check some of the homebrew forums for ideas.