Ha again!
Turns out, SooperLooper builds and installs without a hitch using this method on a clean install:
git clone https://github.com/moddevices/sooperlooper-lv2-plugin.git
cd sooperlooper-lv2-plugin/sooperlooper/
make
sudo make install
cp -r /usr/local/lib/lv2/sooperlooper.lv2/ /usr/local/modep/.lv2/
Loopor also works easily. It has a few extra features (present in the standalone SL, it sounds like), including threshold detection and dub mode. But it doesn’t have a GUI and doesn’t even give access to its buttons from the pedalboard. (Would probably still work through MIDI).
Was also able to install a whole lot more plugins from the MOD Plugin Builder repo, using the Docker Toolbox. Reported in another thread.