My proposal is somewhat the sum of all of your ideas. Well I propose there should a STORE_STATIC_FAST opcode that stores a static variable. Static variable will be declared only once and will be initialized to None (statement syntax will be similar to that of global). It will be initialized in MAKE_FUNCTION. Now it will be set by STORE_STATIC_FAST. Where will the variables be stored? It will have references in locals and __statics__. Therefore LOAD_FAST can find it. So I don't hope there will be performance decrease but performance increase is also not guaranteed. :-)
And if these are thread unsafe then is __defaults__ also thread unsafe?