Some C5 addons already use things like YUI and prototype, so I am sure you could get moo to work. You would need to be careful about namespacing, closures and compatibility mode because whatever you do with moo for your own code would have to coexist with Concrete5 and addon's use of jQuery. You may have to mess about a bit to get the the lazy loader to work.

If you are developing for the marketplace, personally I would stick with jQuery because you can't be sure of what else your marketplace customers will be using and consequently what could conflict.

I like jQuery and am sticking with it. My only hassle with jQuery is that like Joomla it is a victim of its own success with a mass of poorly written and unsupported plugins making it hard to find the ones that are really good, supported and well written.