One of the possible errors that I think Powers has made is that the Herzwesten beer is identified as a bock. Technically speaking, bocks are made using lager yeasts, which were not discovered and harnessed until much later than the action in the book, and certainly well after the period during which the beer had to have been brewed to sit so long
I think that in our attempt to create a brew to celebrate the Herzwesten, we should think in terms of a dopplebock for the assembly of ingredients, but treat it like an alt for the purposes of creating the brew. This would mean an ale yeast for starters, and would likely mean that the hops would be added for the whole boil rather than setting some aside as aroma and/or flavor hops.
Even more interesting, the brew would also pre-date the German Purity Laws. That being the case, we might consider adding things that would normally be avoided in a typical German. The question is, what? What sorts of things can you imagine would have been available to medieval Austria, were sufficiently rich to warrant inclusion, and would improve the character of a dopple-alt?
Think on these things and get back to me.