First issue was setting the environment variable LD_LIBRARY_PATH to the location of the SDK installation. On the Mac this only worked by browsing inside the PicoSDK.frameworks file to determine the full path to the pylib file. On my Mac this turned out to be "/Library/Frameworks/PicoSDK.framework/Libraries/libusbtc08/"
However the demos still didn't work, giving the error message "picosdk.errors.CannotFindPicoSDKError: PicoSDK (usbtc08) not found, check LD_LIBRARY_PATH".
After digging into the Python framework libraries, it appears the environment variable should be "DYLD_LIBRARY_PATH", not "LD_LIBRARY_PATH".
Setting the environment variable DYLD_LIBRARY_PATH to the full internal framework path worked for me.
Hope this helps someone