I'm not certain, but while you're a Delta, you wear ONLY a chapter guard. TCS's wear a special guard, as does Executive Council.
When I called EO and mentioned an idea that I had for a new guard, I was encouraged to do so. I think that age brings privilege!
In that case, you could certainly use a crest!
BTW, when I graduated, we were told to take OFF our guards, as we were now members of the entire system, not just that chapter. I think that thinking has gone by the wayside - I hope it has!