Assuming you don't want to preserve the shadows (or that the shadows can be raised without destroying the image), you can try a variation on the Freemask system (which, with some Hensel strobe equipment, is semi-automated).

Essentially, it involves taking two images. The first would be the "normal" image, as you have here. The second image is taken with the subject still in place and all camera settings (focus and aperture in particular) the same, but with only the background lit. As long as your subject isn't almost perfectly transparent, that second image can be manipulated (using levels/curves and a bit of painting to fill in the holes) to form a good silhouette of the subject, but with increasing gradation along softer edges. You can use that silhouette image as a mask on an adjustment layer, allowing you to raise the background without touching the subject, or you can invert the image and use it as a mask on the subject layer to knock out the background.

It's a few extra minutes at shooting time, but it saves a lot of time in post - particularly if you have a subject that doesn't have sharp, easily-masked edges. And if you have a lot of spare cash floating around, you can automate it (as noted previously) with Hensel flashes and wireless controllers that toggle channels between exposures. (If you have even more money laying about doing nothing, there's even a two-camera rig with a beam splitter and a delay box that will let you do it with live-action subjects. It looks like a real pain to set up initially and doesn't even pretend to be cheap, but if you're doing production catalogue work for good bucks...)