Can Google Docs be used to “sanitize” a Word or Excel document?

The strategy you propose might indeed break malicious code, but with 0 day exploits there is no way to be sure.

If it is just the flat text that you want, the safest way to go might be to open the raw file with a different program. .docx can be opened as a zipped file (using unzip, 7zip or similar) and then you can open word/document.xml with your favorite text editor.