This should be doable using SGPLOT procedure with a "dummy" scatter plot statement and sashelp.class. The dummy plot is needed to allow the plot output to be created. Transparency=1 can be used to suppress any actual plot display. Other options can be used to suppress axes and borders.
Then, use one or more LEGENDITEM statements to define named legend entries you need, including color, type of item and the associated text. Then, add the names of the legend items to the KEYLEGEND statement.
You can do the same with GTL too.
See use of LegendItem in this example:
https://blogs.sas.com/content/graphicallyspeaking/2017/10/27/legend-items/
Also see documentation:
https://documentation.sas.com/?cdcId=pgmsascdc&cdcVersion=9.4_3.5&docsetId=grstatproc&docsetTarget=p0hc32jtfqoy1fn1h5i38q8v9py2.htm&locale=en
https://documentation.sas.com/?cdcId=pgmsascdc&cdcVersion=9.4_3.5&docsetId=grstatproc&docsetTarget=p0xmbppzx71smbn1203aaif96z86.htm&locale=en
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