When doing an XCode AdHoc (or any custom) build in an XCode workspace, Xcode cannot find the other projects in the workspace. To fix it do the following:

gistfile1.txt

see: http://stackoverflow.com/questions/8523690/xcode-custom-build-configuration-causes-library-file-not-found-for-static-libr

In the project with the Adhoc build configuration, override the "Per-configuration Build Products Path" (CONFIGURATION_BUILD_DIR) and "Per-configuration Intermediate Build Files Path" (CONFIGURATION_TEMP_DIR) for the Adhoc build configuration to use the same folder as the release configuration.

Adhoc: CONFIGURATION_BUILD_DIR = $(SYMROOT)/Release$(EFFECTIVE_PLATFORM_NAME)
Adhoc: CONFIGURATION_TEMP_DIR = $(PROJECT_TEMP_DIR)/Release$(EFFECTIVE_PLATFORM_NAME)
Now when you do an Adhoc build, Xcode will put libFoo.a and Bar.app in the Release-iphoneos folder. The linker will be happy and you can use -force_load $(BUILT_PRODUCTS_DIR)/libFoo.a as usual.

Alternatively, you can add the Release-iphoneos folder to the library search paths for the Adhoc build configuration:

Adhoc: LIBRARY_SEARCH_PATHS = $(inherited) $(BUILD_DIR)/Release($EFFECTIVE_PLATFORM_NAME)
But then you'll have to set a different -force_load for each build configuration:

Debug: OTHER_LINKER_FLAGS = $(inherited) -force_load $(BUILT_PRODUCTS_DIR)/libFoo.a
Adhoc: OTHER_LINKER_FLAGS = $(inherited) -force_load $(BUILD_DIR)/Release$(EFFECTIVE_PLATFORM_NAME)
Release: OTHER_LINKER_FLAGS = $(inherited) -force_load $(BUILT_PRODUCTS_DIR)/libFoo.a