Okeedokie! That took care of that!
I missed that you can do this without root privileges on the tablet.
Everything is smaller and sharper now and Roon starts repeatably in tablet mode. (about two dozen starts over three days now) I checked any apps that the change in DPI might have affected and nothing seems worse for the wear. If anything, video performance of UI animations might be a little improved. Movies are sharper (the tablet is now almost exactly 720p). I had to change some font sizes and and reload the wallpaper. All very painless. There some missing characters from time to time in the small type in Roon, but nothing life threatening.
For benefit of visitors from the planet Search Engine and archival integrity, the thread that Danny directed me to (Lenovo Yoga Stuck in portrait) links to http://www.androidbeat.com/2015/07/how-to-change-dpi-of-android-device-without-root/ which has all the particulars on using ADB. The ADB command I used was:
adb shell wm density "160” && adb reboot
Wherein you put the value you want where “160” is.
The command to back out of the change is:
adb shell wm size reset && adb reboot
Again, all the particulars are on that linked page and in Danny’s posts.
When it started looking like installing the ADB driver on my virtual Windows 7 machine might entail some work, I just grabbed the package android-tools on my Fedora Linux box. (You don’t have to install the drivers on most Linux distros, apparently.) With android-tools installed, you can issue ADB commands right in a normal terminal, as a normal user. It worked totally without drama.
@danny Thank you and feel free to link or move this post wherever you feel it might do the most good.