Currently, I don't believe this functionality exists, however, this might be a good one for the Ideas section!
For reference in case you need to do this, here is a workflow I eventually figured out, with the help of a friend who is an art director at a graphic design firm:
1) Print worksheet from Tableau to PDF
2) Open PDF in Acrobat or Illustrator
3) Save As Encapsulated PostScript (.eps) file
4) Open .eps in Photoshop; it will prompt for what DPI to use when rasterizing
5) Edit in Photoshop: if the worksheet was the whole figure, this will just mean cropping out the rest of the page
6) Save as TIFF without layers, using LZW compression (this reduced file sizes from ~50MB to ~750KB)
Unfortunately, this does require Adobe CS for Acrobat and Photoshop, but some equivalent process might be doable using free products like GIMP. But it allows for a custom and arbitrarily large DPI without screenshotting or loss of quality.
The above notwithstanding, more control over export parameters would be a great feature for future versions; I would love to avoid this whole process!
Other option if you choose to export as .emf from Desktop, is to use IrfanView, open the .emf file, resize/upscale as you wish and save it as raster (.png / .tiff)
Useful image processing tools are:
o imagemagick (command line excelent tool)
o inkscape instead of illustrator
o gimp instead of photoshop
o scribus instead of indesign
For future ref, since I just had to work this through: Save the Dashboard as EMF, right click the file in Explorer and select 'Convert to Adobe PDF,' then open the PDF and under the File menu select 'Save as Other...', select 'Image->TIFF.' Click on 'Settings' at the bottom of the dialog box, and at the bottom of the new dialog box, select the resolution you want (usually 300 dpi for publication). Save as, and voila.