How to store an OpenGL program object's uniforms per-context instead of per-program?

Question

I'm working on a multithreaded OpenGL compositing engine. I have a set of shaders which I'm sharing among multiple contexts. For performance reasons, I'd like to avoid compiling a separate instance of each shader program for each thread.

But, if multiple threads happen to use the same program object and I try to set different uniforms on each thread, the uniform values get mixed up since uniform values are stored with the shared program object.

So far I see my options as:

1. compile a separate program object for each thread (which wastes time and resources), or
2. for each shared program object, serialize the process of setting uniforms and rendering (which largely defeats the benefits of multithreading)

Or is there a better way? (I.e., is there a way to store an OpenGL program object's uniforms per-context instead of per-program?)

Edit: I need to support users back through Mac OS 10.6 for a few more months, so until then I'm stuck with OpenGL 2.1 plus whatever extensions are available on Mac OS 10.6. ARB_separate_shader_objects appears in Mac OS 10.8, if I understand correctly.