Adding support for Text Services would enable users of assistive technologies such as Speech Recognition to edit code in Notepad++ much more easily. Without it, speech recognition software cannot be used to select or edit existing text in an edit control.
There are very few source code editors which are voice accessible and I don't think any of them have the features and usability of Notepad++.
Below is an article about implementing support in Scintilla, I thought it may be of some help. Demo code is available to download at the top of the page. I would try and do it myself before asking here, but I know nothing of C++ or Win32 programming.