Thanks to some timely help from jrandall, I now have biblatex-historian properly installed and working, and it is just about as awesome as I had hoped it would be. The historian style has support for archival sources (!): using two types of custom entries, it allows you to cite whole collections easily, as well as cite documents within those collections, that crossreference the whole collection! There is also a special @letter style for—you guessed it—letters, which also can crossref archival collections. No more overuse of @misc! (See the manual for the full sweetness.) Thank you, Sander Gliboff! And thank you, jrandall (and congratulations on being all doctored up!).
The perks of getting jrandall’s help: I ended up with a sweet new xelatexmk makefile, which runs xelatex and biber (now my biblatex backend) the appropriate number of times automatically, so that I don’t have to do that processing myself. I also updated my TeXLive installation, and straightened out a bunch of path and location problems I’d been having, due to the fact that I had basically a TeX rabbit warren in terms of where all the installs were located, what programs could find what source files, and all the issues you get when you’ve been USB-duplicating your laptop for three iterations (meaning I had various TeX installs dating back to, oh, 2003…). Actually, credit where credit is due: jrandall fixed all this, while I cooked a taco dinner for nine people and made sure we all had a margarita in hand. That makes the TeX troubleshooting go down a lot easier.
The upshot of it all is that I now have a fully awesome bib style that actually implements Turabian properly, without a million kludgey fixes and workarounds and improvisations on my part. I highly recommend biblatex-historian to any TeX-savvy historians out there. (And for those of you writing your dissertations in Word: I am so, so, so sorry.)