For an OS X Framework you should be able to use the “Extra Frameworks” entry in the Xcode exporter, if you need to add a framework search path either use the custom Xcode flags in each configuration by setting FRAMEWORK_SEARCH_PATHS=/path/to/my/framework or in the exporter I think you can do -F/path/to/my/framework in the extra linker flags, that’s basically all the FRAMEWORK_SEARCH_PATHS setting does anyway. I also suspect for the framework you can do -framework MyFramework in the extra linker flags but that should be what “Extra Frameworks” does for you anyway.
For including the headers I would indeed make a common bridge, if possible I would try to include
<juce_core/juce_core.h> and use the
JUCE_MAC definitions, if that’s problematic for whatever reason, for windows use…
#if (defined (_WIN32) || defined (_WIN64))
and for OS X use…
if defined (__APPLE_CPP__) || defined(__APPLE_CC__) (warning: that will compile for iOS too, check
TARGET_IPHONE_SIMULATOR for detecting the difference between iOS and OS X / macOS.
Hope that helps.