Window_Base already does something similar, when you're using \c[n] to change colours. But you can change it anywhere in a line, and change it back again anywhere in a line.
You could take a look at process_escape_character in Window_Base to see how it handles colour change and reset based on that.
Window_Base.process_character has logic that detects the start of a new line.
So you could maybe add something to the process_new_line method to turn on a 'new line' flag, then at the end of the process_character method, if the character is NOT a new line character, turn that flag back off again (and reset the text colour if necessary). And detecting your symbol as the first character would depend on how you're going to implement it. If it's just a regular character without the \, you could add something to process_normal_character to say if it's the special character and newline is on, don't print, but change the colour instead; if you're using the \, you'd add it to the process_escape_character method.
