I'm sure at this point I am just re-iterating viewpoints you have already heard, but i would also strongly recommend you do
not edit the existing launcher. Without even thinking about the potential legal issues which i am not really well advised to discuss: Think about the end user experience, Think about the person who wants to use your mod. If your launcher replaces the game's one, that's providing a hassle when the time to uninstall your content comes around isn't it?
If it doesn't replace anything it's just a superflous extra screen for the sake of a button which could easily be a batch file instead - and will have to then start up the proper launcher anyway - at least for steam versions of amnesia.
Which brings me on to the second point - you are just asking for compatibility issues, in areas where FG has already set up the game in such a way so that you don't have to worry about them.
E.g. The steam version of the launcher probably behaves differently (you can launch a non-steam version without going through the launcher, so I'm guessing the launcher does something special in the steam version). Are you going to reverse engineer the different launchers to add your button? How many different versions of the mod are you planning to maintain and upload?
Then there is the issue of other parties as well as other custom stories:
- What if you release a second story which also adds a button in the launcher? Which launcher do I keep - Do i waste space and keep both?
- What if other people decide to jump on the bandwagon? I'd rather the full conversion launchers be in the full conversion's folder.
- What happens if FG release a patch for amnesia in the future which alters the launcher?
Finally, why should the user
want to start your CS/FC differently to all the others? All the Custom Stories are in the custom stories menu - where the user expects them to be - each in their own folder to make it easy to remove too. Full conversions are generally self contained too, and have a short-cut or batch file to launch the game.
I really think it isn't necessary making this extra work for yourself when all you are really going to achieve annoying the end user. I ask
what do you think adding the button to the launcher is going to add to your mod that a shortcut or batch file can't?
I apologise if I am coming across as a little blunt here, but if you are trying gauge feasibility, then this should give you some more counter points to consider.