I vote for a sticker to be issued to all competitors in addition to the shirts (hey we all want our $450 worth right?), something along the lines of "SUPERLAP TIME ATTACK" or even "SUPERLAP TIME ATTACK COMPETITOR"
For every event entered you add another stroke until 5, then use a strikethrough for the 5th, then start again, eg:
EDIT: Start at the bottom and work your way up - sorry