You can't log to the file when it is opened in excel in default mode, because excel will lock the file. You can first copy the file and then open the copied file in excel. This will leave the original file unlocked for logging. Notepad will not lock the file when it is opened.
However you can also make excel open the file as readonly with the /r switch:
EXCEL.EXE /r <your file name>
And you can change the default in your registry:
[HKEY_CLASSES_ROOT\Excel.CSV\shell\Open_in_read_only_Excel]
@="Open read-only in Excel"
[HKEY_CLASSES_ROOT\Excel.CSV\shell\Open_in_read_only_Excel\command]
@="\"C:\\Program Files (x86)\\Microsoft Office\\Office12\\EXCEL.EXE\" /r \"%1\""