I basically agree with everything Selket says.
As a CM I am usually very busy. I can usually (under certain circumstances) take the time to take a photo for guests. Sometimes as a Guest I have been asked by other Guests to take their picture. However, while I have their camera I am clearly in their sight.
I'm not in attractions, but I have been in positions where I have had Guests ask me to keep an eye on something for them. Depending on what it is and the situation I might agree, but I clearly state that I cannot accept responsibility. There are too many possibilities that can come up which will divert me, the least of which is under the Cast Deployment System someone may come up to me and tell me that I am now scheduled for a 15 minute break.
There is no way any CM would sign a paper accepting responsibility for several reasons. One is that nobody at the operational level has the authority to accept liability on behalf of the Company, and if someone does and it is found out that is cause for termination. The other is that $5,000 is betweeen three and four months total pay for full time employees.
Also, a rider on your homeowner's insurance would be similar to a jewelry rider and not cost much.