I'm posting here on the theory (and also based on various threads I've seen here & around the internet) that I'm not the only PD user who is interested in using TouchOSC on an iPad to control PD on a Mac over a stable, wired USB connection rather than a sketchy ad-hoc network (which can suffer latency & dropouts when used in venues/areas with wifi interference, and prevents TouchOSC from being a reliable live performance interface).
Most of my searching turned up the same answer: networking over USB is not possible without jailbreaking the iOS device & installing a tethering app (I don't have the option of jailbreaking due to compatibility concerns with other apps I use professionally). I also discovered a free app called midimittr which creates a MIDI bridge with a server application on the OS X side via USB, which is great, but alas my control setup is dependent on OSC messages rather than MIDI.
Now for the (almost) exciting development: far down in search results I learned of the $7 Midimux app/server, which promises both MIDI & OSC connectivity directly via the standard Lightning=USB cable, no jailbreaking required! After buying the app, installing the server, configuring my firewall, checking port settings on all sides—I was thrilled when I was able to send OSC messages from PD to TouchOSC on the iPad! But...... no such luck in the other direction, TouchOSC cannot send to PD. I've tried a variety of port/hostname/IP settings, and fiddled with all of the OSC/UDP receive objects in PD to see if any connection is being made in the iPad->Mac direction, with no success. The MIDI part of the app seems to work just fine both ways.
I am not proficient in advanced networking, so I don't have any other methods for testing to see what the issue may be. I also realize that the problem almost certainly lies on the Midimux/TouchOSC side of things rather than with PD, but as mentioned before, I figure there must be other PD users who would be happy to learn of the potential for a wired TouchOSC interface, and it seems the possibility may be tantalizingly close. If anyone has any thoughts/experiences/suggestions on the subject, I'd be delighted to hear them.