I can't seem to find a way to do a scripted Export that fixes the file type to Excel, without providing a Path. If I leave the Output File Path list blank, the FileType: setting won't stick, always re-setting itself back to text.

I realize I could export to a known filename then rename the file, but I'd rather let the user choose the file location themselves (but not allow them to accidentally get a text instead of XLS export).

There is a script step 'Save records as Excel' which you should use rather than doing a traditional export of data. That way you can let the user choose the folder as you require.

It should be noted that your layout should reflect only the data that you wish to export to get the best out of this feature.