Interposing functions calls from static library to system framework - c++

Question

I have a situation in which I need to interpose function calls made by a third-party static library to an iOS system framework (a shared library). Chances of cost-effective or timely maintenance support from the vendor of the static library are slim to non-existent.
In effect changing the calling sequence from:
+-------------+ +-------------+ +---------------------+
| Application | ---> | libVendor.a | ----> | FrameworkA (.dylib) |
+-------------+ +-------------+ +---------------------+
to:
+-------------+ +-------------+ +------------+ +---------------------+
| Application | ---> | libVendor.a | ----> | Interposer | ----> | FrameworkA (.dylib) |
+-------------+ +-------------+ +------------+ +---------------------+
The orthodox solutions to this problem is either to persuade the run-time linker to load a different library in place of FrameworkA or loading libraries with dlopen(). Neither of these are options on iOS.
One solution that does work is using sed to rename symbols in the the symbol table of libVendor.a.
Suppose I want to interpose calls FrameworkA_Foo() in FrameworkA made by functions in libVendor.a:
sed s/FrameworkA_Foo/InterposeA_Foo/g < libVendor.a > libInterposedVendor.a
And then interpose it with:
void InterposeA_Foo()
{
// do some stuff here
// ....
// then (maybe) forward
FrameworkA_Foo();
}
This works just so long as the length of the symbol names remains the same.
Whilst this approach works, it lacks elegance and feels fairly hacky. Do better solutions exist?
Other approaches considered:
Changing link order in order to get the interposing function to link to libVendor.a rather than the framework: Apple's linkers (unlike those on most UNIX platforms) recursively resolve symbols in libraries, and the order they are presented on the command line makes little difference)
Linker scripts: Not supported by lld
mach-o object-file editing tools: Found nothing that worked
[For clarity, this is C (rather than Objective-c) API and the toolchain is clang/LLVM. Using an Apple-supplied GCC is not an option due deprecation and lack of C++11 support]