To be perfectly frank, this proposal seems far more about creating the appearance of safety than addressing an actual deficit in application correctness. I'm not questioning the value in detecting unhandled promises (resolved OR rejected) as a development tool for calling attention to a potentially undesired flow... but just like other lint rules, this belongs in tooling and NOT the execution environment.

valac produces C source and header files from Vala source files as if you've written your library or application directly in C. Using a Vala library from a C application won't look different than using any other GObject-based library.