Is it possible to write the results (with ex. xls writer) using variables from loops? It would be useful in my opinion. (And actually if the xls writer could append sheets to existing ones would be also nice feature.) If it is not possible, I would recommend adding new nodes to add this functionality, or make the loops execute the nodes with no outports (but input is between the start and end nodes).
Maybe you could give some concrete example to make your case clearer.
However, if I understood correctly, you can definitely use variables to write results. Just perform the write action within the loop.
Regarding, the appending sheets remark, you can use the excel sheet appender node to accomplish the appending of new sheets to existing files.