The end goal is that I want to show part volume with units on a drawing.

The problem is that I don't think you can determine the part units from the drawing.  If you use "Volume" as a custom property, and call it from the drawing, the number shown will depend on what units the part document is set to.  The same code could be put on the drawing: ''SW-Volume@Part1.sldprt'', with quotations, for the same effect. Again, the number displayed will depend on the part's units.

So if you want the volume shown on a drawing, with the same units all the time, how do you do that regardless of what the part units are set to?  Maybe the units would have to be called out manually, like "SW-Volume@Part1.sldprt" cm³, for example. But as long as you're getting a certain unit, that should be fine.  Again, the example in the previous sentence can't be used because it isn't reliable. If someone changes the part units, the noted volume is incorrect.

Has anyone dealt with this before? I couldn't find any other discussions about this specifically.

Thanks.