Hi angel :
I think your idea is very interesting !,here is my answer hope you will find what you are looking for :
You may build an application as a service (or hidden process) which its mission is to hook on every window (using API functions) and convert any Text box to password char ..
So will need :
1- API functions (a C# wrapper version ) :
FindWindow,SetWindowParam ...so on
2- Notify icon on tray area that pauses the whatever do to the process.
start here and use the idea not C++ code ! ;)
well let me know if this works and I'll be glad..
best wishes and good luck ..