It is not against the rules to compete with a finished dog.  In some of the rarer breeds, you sometimes see finished dogs used as fillers to build majors.  But showing the dog in the classes at the national after it finished does seem unsportsmanlike.

I was taught that if pulling your (finished) dog would break the major (not an issue at the national) then you should not bump it up.  Or at least you should check with your competitors.  I recall years ago being asked to show a bitch who only needed majors.  If I had pulled her, the 2 points would have broken to 1.  I consulted several competitors before leaving her in.

I proceeded to win the two points and take BoW, so not even to dog got an extra point.  I then stated "That was it."

She was not shown again unless there was a chance for a major.