Can we make isoglosses?

[Anton]

The Voll-mich reference is clearly linked to Vollmilch.

It does, and, moreover, it specifies the pronunciation (which excludes the possibility of a typo), but what confused me it does not provide the meaning but just links to an altogether different dictionary's article of "Voll-milch" (not Voll-mich). I was not able to quickly locate the scanned version of the Rheinisches Wörterbuch to check it first hand.

Even more interesting if you search 'mich' you find two expressions:
Kaufmich -> merchant
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Zwingmich -> stubborn person
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Indeed, what I did yesterday I used the wildcard search (supported by Woerterbuchnetz) to look up *mich. What attracted my special attention was the AHD "chasmich". The article for that says "see kamich", but then unfortunately there is no article for "kamich", except for in another dictionary where it means white mold on liquids, which is probably irrelevant. It is still not clear whether that relates to "chasmich" or not.

In both cases '-mich' stands for Michl which stands for -guy
While not Austro-Bavarian at all, it could indicate 'gaismich' in the Herzogenburg document is indeed the goat-guy which would make sense regrading the reference in the St. Pölten Urkundenbuch for 'Geisiunge gans' - goat-boy. It would mean the same thing. Geese to be paid for the services of a goatherder. Though 4 geese per year for a goatherder seems excessive.

It also doesn't make sense in the VM-context as goatherder

This may be promising. The amount of geese per year would probably depend on the size of the flock, not on the goatherder's appetite. Regarding the VM-context... it's still at odds. "Goatmilk" fits the VM-context, but not those legal contexts. On the other hand, the goatherder is at least possible in the legal contexts, but does not fit the recipe context. However, it must be one of the two - or some altogether different thing.

However, I would not dismiss the goatherder offhand. It's often forgotten that the phrase is not "so nim gas/mich", it's "so nim gas/mich o" (whatever the "o" stands for). Now, as I commented just yesterday on Koen's blog, one of the prominent "so nim" contexts is 3 Kings 19:4:

Es ist genug, so nimm nun, HERR, meine Seele

Where "HERR" (Lord) is the one who is addressed here. What if a goatherder is the addressee of the VM's "recipe"?

Blah blah, so take ye, the goatherder, that "o" stuff.