I suspect eutectic liquid InGa alloys would work, they stop wetting the glass when you are under vacuum. Although mercury is safe in the tube, just don't break the glass.

To put it in perspective, people used to consume mercury-based medicine. I'm sure if the glass is thick enough, the risk of a spill (and your health) will be generally managed.

Just drink a little bit at a time to buoys up an immunity.