Great replies! Thanks.
It never occurred to me to wear the hood or a shower cap under my helmet, but I can see the advantage.
Seems like some kind of loop/hold-down would be needed to prevent the cape from flying up and into my face. I will check out the "hand loops" as mentioned by BobbyG.
I'll probably go with an inexpensive cape as I usually have the option of not riding in the rain. However, it would be nice to have one handy when on a multi-day tour.