This github repository contains a polyfill of the DOM parts proposal under /src.
The polyfill does not add any MutationObserver that listens for node additions or removals to update the list of parts in PartRoot objects. Instead, every call to getParts(), partRoot, and valid does DOM traversal.
There is however cached equivalents of most methods, such as getCachedParts(), that returns the result of the last DOM walk, if the user knows that the DOM has not been mutated since the last DOM walk. There is also a setCachedParts method, which you should use at your own danger since there is no validation that the parts are actually valid. Using these methods may leave the Part object in an incorrect state, but any call to the actual API methods will return the correct result for what is actually in the DOM.