I rather like those suggestions! A red cross is definitely something that is pretty universal. However, I do have to agree with Beguile's assessment of using a halo or angel wings or a lily.
This may be morbid, but your suggestion brings up something I've thought about before. I'm a member of several forums, all ranging in subject matter and purpose, and I've made some great friends at each one. So, in our firesafe, along with important documents like birth certificates etc., there is an envelope with a list of websites and log in information and instructions on each for who Mr. S needs to contact if something ever happens to me. There's another sealed envelope with the same for him.
Does anyone else have something like this, or am I just weird?
Anyway, end hijack, I'm sorry for that....