What about the Protestant Knights Hospital (also known as the Johanniterorder)? The Knights Hospitaler in Malta built a hospital which was, according to sources I’ve read, open to all comers regardless of religion. Since the Protestant orders were, through an odd quirk of fait, still subordinate to the Catholic grand master in Malta at the time and since the only real thing they changed during the Reformation was their theology, it seems likely to me that they’d fit the bill for a sponsor.
Of course, they might not be in Magdburg at this point, but I find it hard to believe that Mary Simpson hasn’t run into them.

I must disagree with your translation in paragraph three, however.

If you will refer to the following citations, you will see that “Frauen” is not the modern German word for “women”, but is instead a possessive form equivalent to “belongs to a female aristocrat”. This may be translated incorrectly by an up-timer.

However, the word “liebenden”, not the word “lieben”, is the equivalent of the English gerund “loving”. Up-timers would understand “lieben” to be the German word for “beloved”. For example, “Lieber Herr” is often translated as “Dear Lord”. If up-timers translated “lieben” as the infinitive form “to love”, then “to love Lady/Ladies” is a translation that is almost correct.

Therefore there are several plausible ways for up-timers to render the name of the facility, both correctly and incorrectly.

“Cloister/Monastery of Our Dear/Beloved Lady/Ladies”

Americans and Spaniards will recognize the name “El Pueblo de Nuestra Señora la Reina de los Ángeles del Río de Porciúncula” as one version of “our Lady” and the French would recognize “Cathédrale Notre Dame de Paris” as another form.

Thank you for creating and extending this wonderful body of work.

Astrid

http://de.wikipedia.org/wiki/Kloster_Unser_Lieben_Frauen
http://de.wikipedia.org/wiki/Unsere_Liebe_Frau
http://de.wikipedia.org/wiki/Frauenkirche
http://de.wiktionary.org/wiki/unsere_liebe_Frau