You could add in the glogin file in $ORACLE_HOME/sqlplus/admin something like this:

set serveroutput on size 1000000

set trimspool on
set long 5000
set linesize 100
set pagesize 9999

column plan_plus_exp format a80

column global_name new_value gname
set termout off
select lower(user) || '@' || decode(instr(global_name,'.',1),0,global_name,
substr(global_name,1,instr(global_name,'.',1)-1)) global_name from global_name;
set sqlprompt '&gname> '
set termout on

This will work the first time you connect to sqlplus

Cheers

Angel