In the making of wine (which we need to think of when we're talking about mead), grains are not an allowed adjunct. It is for this same reason that maple syrup is not allowed in the production of wine (or in our case, mead). Though I'd love to see how the folks at Sap House meadery in NH got around that one.
Anyway, the way to get "around" that law is to make a beer, or better yet, a "malt beverage". A winery can NOT produce malt beverages, so a company that wants to produce wine and beer (or malt beverages) has to also have a brewing license. The facility can be bonded as an Alternating Premises, but the TTB (Federal) does not really like to give that one out, and one would have to go back and forth with them for a while (or at least, that's been my luck).
Once the facility is setup as an alternating premises, the company would have to notify the TTB each time the production facility changes over from beer to wine and the other way around. I do not recall how the finished product had to be stored (separate areas or combined), but I don't believe that the government cares too much about the Tax-Paid area. That area is for alcohol for which the taxes have already been paid, so they don't really care as much about that. A tasting room for example is a type of tax-paid area.
Now, all this being said. There are undoubtedly going to be people in the industry that disagree with at least something I've said. And that's because if the TTB is one thing, it's inconsistent. Many things depend on who is assigned to your application/license, and how THEY interpret the rules.
Brad