ISBNs are of limited value since they were only adopted in 1970. Most of that material is still copyright protected. The public domain material on Wikisource is much older than that, and has no ISBNs.
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Once ISBN is implemented we can expand to LCCN, which is everything copyright in the US library of Congress, and is not a copyright system. From this other call numbers, provided they can be referenced in the public domain, will be possible. ISBN's are not of "limited" value, since anything which has had an edition since then has one. This includes many texts which were produced prior to 1970, and virtually all of the commonly cited material on wikipedia. Since the proposed format allows for different types of identifiers, it will be a simple matter to add additional identifiers by which material may be cited.