Is this even technically possible? How big of a bounty on bountysource it would take for someone to tackle this change? :-)
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It's entirely possible.
Remove Fx (and Element.Styles) from your build, include moofx, include the adapter, and it's done.
The adapter for mootools: https://gist.github.com/kamicane/2238266#file-moofx-mootools-js
But I don't think there is any way to keep it backward compatible.