I am trying to document the use of the word "aus" meaning "from" to the 8th Century AD in the area around Aachen. Are there any English language documents I can reference - or is the word a more modern construct?
I doubt that aus is a modern construct per se. Two unrelated things seem to be at play here.
Modern High German aus has many close cousins in other Germanic languages like obviously English out or Dutch uit, and it is loosely cognate to all, in the sense of stemming / progressing / made of. It is also a classic German sound shift example from (plosive) /t/ to (fricative) /s/.
I would speculate cautiously that the likes of aus, out, uit go back even further to some common proto-Indo-European root, without which the origins of the Slavic cognate iz and even Latin ex become hard to explain. But I could be wrong.
What runs counter to these commonalities is that etymologically related prepositions are semantically unstable from one language to the next, and the example you cite is inadvertently telling of that instability. There is little rhyme or reason why English 'out' is not habitually used when describing geographic origin, but German 'aus' is.
Thus, English I am from Germany / Austria / Switzerland or Dutch ik ben van Duitsland / Oostenrijk / Zwitserland does indeed become German ich komme aus Deutschland / Österreich / der Schweiz. An English speaker's use of out of in such contexts would perhaps serve to render emphasis but is otherwise an anomaly... 'I am out of so-and-so (meaning I won't be back any time soon)'. When moving to Western Canada from the UK I had to get used to people here saying that 'so-and-so company is based out of Calgary', which sounded unnecessarily emphatic to my ears at first but is ultimately just local idiomatic variation. Er... what are you now, still based here or out of here (hence no longer in business)?
Appropriate usage of prepositions is therefore a matter of hefty rote learning in each new language. There is no logic whatsoever why you dream of and think of something in English but soñar con (with) and pensar en (in) something in Spanish... or träumen von but denken an in German, for that matter.
There's a reason I bring the Dutch language into the equation:
The vernacular speech of the Aachen-Maastricht area is part of the Limburg-Ripuarian language variant and Frankish in origin. I grew up in Köln, so to me some kinship between the neighbouring Maas and Rhine dialects is audible even tough I'm not fluent in either. Since Limburgish dialects have been under an (Amsterdam) Dutch standard-language umbrella and Ripuarian ones under a (Prussian) German one for 200 years, chances are that the grammatically appropriate usage of 'van' and 'aus' in the respective 'prestige' languages (of schooling etc) has had an imprint on these dialects over time. Conversely, (Western) North American based out of may be an archaism; a time capsule of what was once common in dialectal English but has fallen into disuse in British RP and is now considered nonstandard.
So the boundary between West Germanic (EN, NL, DE) prepositional usage of aus/uit/out versus von/van/from may once have been significantly more fluid and only 'froze' roughly at the onset of the Industrial Age. That makes me unsure as to how informative individual written sources are unless you scan a significant number, pinned scatter-plot style to precise dates and places of origin.