Yes, you can. It depends on your setting of "aaa authentication local-override" where you have to change it. With local-override, the Cisco has to provide the initial Username: prompt. Beginning with 11.1(4.1) (CSCdi59376), this prompt is configurable by "aaa authentication username-prompt".
Without local-override, the prompt is provided by the TACACS+ daemon. Whether it is changable or not depends on the daemon implementation. For the Cisco PD daemon, the source code has to be modified in a trivial way.