Today we had an exploration day, which means we can try out various ideas that could be beneficial to the engine, but aren’t necessarily scheduled soon (or at all). (Other companies call it Hack Day etc)
What I tried today was to tie together a test branch we had that @ChristianM had prepared, where we have an extra physics library: physics_null, and the Native Extensions’ App Manifest.
The app manifest lets you modify what libs/symbols/jars etc that should or shouldn’t get linked into the engine.
After some small modifications, I got it to work nicely. I still have a few things to clean up (or think through), but this functionality should arrive in a sprint or two 
My example game.appmanifest:
platforms:
# Null Physics
common:
context:
excludeLibs: ["BulletDynamics", "BulletCollision", "LinearMath", "Box2D", "physics"]
libs: ["physics_null"]
And some before/after results on x86_64-osx:
mawe@physics_null (master) $ ls -la dmengine*
-rwxr--r-- 1 mawe staff 4463180 Oct 20 15:58 dmengine_no_physics
-rwxr--r-- 1 mawe staff 3375996 Oct 20 15:58 dmengine_no_physics_stripped
-rwxr--r-- 1 mawe staff 5619372 Oct 20 15:59 dmengine_physics
-rwxr--r-- 1 mawe staff 4111380 Oct 20 16:00 dmengine_physics_stripped
4111380 - 3375996 = 735384 bytes saved