Hi Ben, there isn't any way for that particular rendering method to generate a transparent background.

But there is another way you could get one - if you do an export to .ai format, and check the option for generating a shaded background image, there will be a companion PNG file generated along with the .ai file and that PNG file will have a transparent background.

Also if you export to PDF format, the PDF will contain in it the same kind of bitmap which you could probably extract with copy/paste.

- Michael