Kerbal Space Program SaveSharingServer is an attempt to make a mod for KSP that allows several players to share their progress in semi-real-time by modifying their save.
The current version allows you to merge any two save files that you have at hand, which in theory means you can do "play by mail" multiplayer. The project uses QT for all of its components, and is written in C++. You can read more about the project, or download a prebuilt version at https://www.daymaredev.com