Answering my own question (sort of). It turns out that my Linux distro (Void) did not build their tvheadend with hdhomerun support. I'm not sure why, since it seems like the most popular ATSC tuner by far. Perhaps it's because the majority of users are outside the US and don't care about ATSC?
In any case, I cloned the source from github and built tvheadend myself. This was fairly painless, but I do have one small correction to the build instructions: You cannot just run tvheadend from the build.linux directory after the build completes; you need to run "make install" first. Otherwise, it complains that it cannot find some data files, and immediately crashes (on purpose).
With hdhomerun support enabled, tvheadend is now seeing my tuners. I would still like to know how to set the IP address manually, because I know there are often issues with multicast packets traversing wired/wireless bridges, and many people may be connecting to the tuner through a wireless AP. AFAICT multicast isn't needed to actually run the tuners, so it shouldn't be a hard requirement. It should be limited to a convenience tool for those that don't want to configure by hand. And anecdotally, this is a fairly common problem.
