@Cliff, those are some fun designs. I personally find spinnakers a fun way to create some visual interest.
I've seen photos of spinnakers from larger boats that had a giant "pink panther" and other designs. Here's a link showing a few other designs, including a gigantic eye:
https://www.pinterest.com/nortonyachts/spinnakers/My personal opinion on class rules is only that, but as long as the design is "fun" and not commercial or intentionally offensive, I would hope that the board would rule that there aren't any obstacles to using it in a class event. But not my call.
Having been on a race committee (not a DS event) on several occasions and for different fleets, I find sail numbers on spinnakers useful. However, the reality is that many low-commitment events attract boats that may sail with used sails, so our race sheets would list multiple sail numbers and/or other identifying marks for each entry. A bit more challenging, but we always managed to sort things out in the end. Digital photography to the rescue: you don't have to get it right "in the moment" but can look back to double check.