Some serial interfaces may allow bi-directional signals on lines other than transmit, receive and ground.
However, you can should never rely on this being the situation. Serial ports are made to function according to the manufacturer and may only work in one direction. And this would fulfill the requirements of most serial devices. I've never seen any software that required DCD to be bi-directional.
are the standard signals. But, hopefully you have something like this already.
I don't see this as a problem with Linux at all. It sounds more like you're trying to do something you shouldn't.