This here is a "mod" that allows modders to add new player ships without having to replace existing ones.
It modifies the ship selection UI.
I figured I'd post this in "Mod Development", since it doesn't really do anything by itself.

The original buttons (arrows, list, A, B, C) are disabled.
You can use the new "previous" and "next" buttons to cycle through all ships whose IDs start with "PLAYER_".
So, if you want to add a new ship without replacing existing ones, all you have to do is give it an ID starting with "PLAYER_".
This also has a side effect where users can access ships they haven't unlocked.
This currently only supports the latest (1.5.13) Steam version on Windows.
Download (Mediafire)
Source code (GitHub)
Note: THIS MODIFIES YOUR FTLGame.exe. If it breaks, you may have to re-install FTL.
Installation
Download and unpack the .rar
Run install.bat and follow instructions
Uninstallation
Run uninstall.bat and follow instructions
If that doesn't work, go to your FTL directory and rename FTLGame-backup.exe to FTLGame.exe
This is actually a sub-project for an FTL multiplayer mod I'm currently working on, which is still a few months away from a public release.