re: Understanding the classical model for linking: You can override an LIB with another LIB, and a LIB with an OBJ, but you can't override an OBJ

Sorry if I've misremembered this.

In printf.c, declare extern (*printf__fp)();

In fltused.c, define printf__fp.

Then when printf wants to print a floating-point number it just invokes printf__fp and you crash if you didn't trigger the include of the floating-point library.

Better still would be a weak symbol if that's supported.