Backup name in Maintenance plan
You would need to build a custom backup script which uses dynamic SQL to build the name of the .bak file. The maintenance plan could execute this with the 'Execute T-SQL Statement Task' item.

The T-SQL would look something like this

Select @SQL = 'BACKUP DATABASE [' + @DBName + '] TO  DISK = N''' + @Path + '\' + @DBName + '_backup_' + @Date + '.bak'' WITH NOFORMAT, INIT,  NAME = N''' + @DBName + '_backup_' + @Date + ''', SKIP, NOREWIND, NOUNLOAD, COMPRESSION,  STATS = 10 '

EXEC SP_ExecuteSql @SQLToExecute = @SQL