Great question, Susan! In full capital case, superscript letters and abbreviations like Jr. should ideally remain in all caps for consistency. However, small caps can be a good choice for abbreviations like "Jr." to keep the text legible and balanced with the rest of the name.

For instance:

JANE MCDONALD (with the superscript letter in normal caps)
WILLIAM DOE, JR. (keeping Jr. in all caps or using small caps for a more refined look).
This way, you maintain uniformity while also giving a slight differentiation to the abbreviations for readability.