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Old thread, but I ran into this today and the feature still seems to be missing.
Only workaround I found is double clicking the dimension, and using the "Replace With Text" as my first line of supplemental text, using the "Above:" field as the dimension itself and using the "Below:" field as my second line of supplemental text. This gives 3 total lines of text / dimension, but removes the actual computed dimension, so any changes to the origin points will not be taken into account.
any way to get the text to justify to the left of right instead of always centered?