I am not able to fully resolve this requirement. However, I have implemented a solution that should simplify the construction of a user ping. Below the editor you find an input field dedicated to insert a user ping. Type there some characters of the desired username and select the user, when it gets displayed below this field. On selection, the ping will immediately get inserted into the editor's text at the last cursor position.
The solution is not yet extensively tested, please send me feedback in case of issues.