I haven't tried this, but the doc says you can overlay box plots as long as you have same category variable. So, I suggest you can have two analysis variable columns A & B. A should have analysis data missing for last category, while B should have data only for the last category. Then, use VBOX with connect with A and a separate VBOX for B. Both should have the same category variable. This will avoid the need to compute the mean values.
If this does not work, you can use the methods described in this blog article to overlay custom connect line.
https://blogs.sas.com/content/graphicallyspeaking/2015/12/04/boxplot-with-connect/
https://blogs.sas.com/content/graphicallyspeaking/2015/12/06/boxplot-with-connect-using-annotate/
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