There should be no problem with carrying national letters as the rules only specify "if required" and when they "shall be required". See the wording below (italics). As far as I am aware there is no rule that says they are not allowed at other events.
4(f) National Letters, if required, shall conform to the
same type, size, spacing and requirements as sail
numbers (refer rule 4(b), (c), (d) and (e)) and shall
be positioned as follows (also see diagram):
National Letters shall be required at all World
Championships, Regional Championships and
events described as international events in the
notice of race or sailing instructions. National
Letters may be required at any other regatta by
the notice of race or sailing instructions.