Hi @jimskeet2002,
As you noticed, these properties are driven by the model units, so without changing them, it will not be displayed in kg to my knowledge. One thought I had regarding your dimensions however - you said that you did not want to enter units in meters but in mm. You can actually enter dimensions in whatever units you need to. For instance, if your model units are in meters, but you want to create a 10 mm line, you can actually just type 10 mm. As long as you have the "mm" it will do the conversion for you.
Thanks,
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James Youmatz
General Adoption Manager | Informed Design