At the time I posted this thread, the github repo in question had all the libraries in a subfolder… and Ivan gave me hints on doing a pre_script that would pull them in and point the LDF at them…
But to do multi-libraires in the one repo, unless something has changed or broken, the magic sauce is the include statement in the library.json … if you look at one of the libraries from jrowberg/i2cdevlib… i.e. the BMP085 library … it is in the Arduino/BMP085 subfolder… BUT only the files in the folder referenced in the include statement in the libraries.json (relative to the root of the github repo) are actually included in the library, thus making it self-contained - you can check the source code at the bottom of the library installation page to see how platformIO interpreted the library configuration settings.