While it is possible to pull some power from the RS232 lines, there are a couple of serious limitations.
First, remember that the efficiency will be lower than tapping the source voltage directly. (There is no free lunch, you always get out less than you put in.)
Second, some software is required. This tapping a control line as a power source, assumes that you have a driver that sets the control lines to the required state. If the control lines are not at the proper state , or other code using this serial port changes the state, you will lose your power source.
Lets say you are using this device with a serial port terminal program that gives you lots of control. It won't be a big deal, as you can set the required control lines high with the terminal program. However, if you want to use this power source with a third party application that you have no control over, it may not work. That third party app, may reset the port on startup and turn off your power source.