Hey GotLag, I have implemented this feature, if you want to incorporate it in your release! All I did was add a remote interface so that other mods can add sounds and assign them to specific entities. They can also assign "none" if that entity should not make any noise. The Honk mod saves the assignments in a table and checks it for the target entity before defaulting to the normal sound.
See the last commit in my fork: https://github.com/robot256/robot256_Honk
Here is my example of how to use it: https://github.com/robot256/MoreHonks
As a test, I added some very nice steam train whistles to the steam engines in Yuoki Industries - Railways and the Steam Locomotive mod. Next I'm going to add some fog horn sounds for the cargo ships. In the future, it would be possible for mods like Cargo Ships to include the horn sounds themselves and register them with Honk to the appropriate entities.
Edit: I posted my fork with the interface, and a new mod with the new sounds. Feedback is welcome!