Currently we create SpreadsheetML and send to our client with response.write(Spreadsheet as a string). We can no longer use XML spreadsheets. We also cannot write files to the sever. Is there a way to use workbook.easy_LoadXMLSpreadsheetFile(Spreadsheet as a string) with a string vs Hardfile, or feed response.write(Spreadsheet as a string) into a loader of some type? The code available for reading an XML Spreadsheet from a file and rewriting a XLS seems so easy.

You can write your SpreadsheetML string to a MemoryStream, instead of a harddrive file:

Use the the MemoryStream with workbook.easy_LoadXMLSpreadsheetFile method and then convert the workbook to XLS with workbook.easy_WriteXLSFile(Response.OutputStream).

Useful links:
Open Excel in browser using Response stream
and
Convert XML to Excel
section "Convert XML Spreadsheet file to Excel"