There is nothing that prevents you to put inline in a static function in a .cpp file.
Some compilers have the option to force an inline function, see e.g. the GCC attribute((always_inline)) and a ton of options to fine tune the inlining optimizations (see -minline-* parameters).
My recommendation is to use inline or even better static inline wherever you see fit, and let the compiler decide. They usually do it pretty well.