Decided to forgo a timely dinner to hack around on the Droid a bit more; after about a half hour of investigation and some minor reverse engineering, I tracked down the details of how the native Android SMS messaging app displays its little message counter (indicating how many messages your text spans, and how many characters remain until you incur an additional message).
It's a slightly hardcoded bit, so it won't age gracefully should future updates to the Android platform muck about with the way the UI looks. However, as for now, it fits in very nicely, and works perfectly. So there's another item to tick off the list.
That leaves the icon (which shouldn't take more than a half hour in Paint.Net to throw something together) and a rather pesky little matter of message histories.
As it turns out, Google decided not to expose the SMS history database via a public API. You can barge in using SQLite and just fool around yourself, but the format of the data isn't guaranteed to be stable across releases, so it doesn't really strike me as a good idea to take that route.
Instead, I think it makes the most sense to just write my own little history feature that shows the message and the list of recipients. In technical terms, that just means hacking up a couple of additional Activities and throwing in some quick UI work to make things look nice. Overall that doesn't seem like it should take more than a weekend to polish off and get to a point where I'm happy with it.
If I can scrape together another hour or two during evenings over the course of the week, I should be able to keep to my preliminary schedule of releasing sometime in the next week or so.