Perhaps you as the insuring party will need act as the client's personal bank account, monitoring all access and transactions in order to properly ascertain legitimate use.

I'm not a liberty to discuss the risks of this service to me until a Terms of Service is fully developed. I've worded the OP to specifically guide you away from considering the how/whats/whys/when of this plan. I have a schedule of risks and costs that I'm working from. What I don't know is how much something like this is generally worth. From that information though, I can judge it if will be equitable to pursue establishing this kind of service.

But seriously, I'd really like you to consider the fantasy that, from your perspective, no Bitcoins would ever be lost. What would it be worth on a yearly term?

Call Lloyds of London. They will insure anything.

http://www.lloyds.com/

To store Bitcoins with near perfect security you have to create a private key on some clean offline system, encrypt it strongly and distribute that encrypted copy over multiple long-term storage locations, print it, deposit USB keys with it etc.
The risk to lose your Bitcoins after you have done that is very near to zero (at least it is probably lower than the risk that any Bitcoin-insurance company goes out of business and cannot longer fulfill its obligations).

Bitcoins are different from other assets because there are no real running expenses with keeping them safe (eg. no secure physical storage space is needed). So I'd say that you can probably charge a one-time fee in the order of 1-2% but only a very minimal annual fee - maybe around 0.1%.

With Bitcoins, the risk of losing them does not really increase over time once they are stored securely so that should be reflected in your rates.

Of course, if your service would not only keep your Bitcoins secure but also available at all times (ie. you can actually use them to make payments all the time) you can probably charge higher fees.

I'm still not interested in any dis-service that guarantees a loss. Perhaps You pay me to keep my own safe ? now that would be worth something... but in whose fantasy ?

The state in it's current form is nothing but a limited liability insurance scheme. The strict liability of common law has been abandoned for statutory and other conditional limits. It's hidden well from the populace.
The state is just forced placed insurance provided at the end of the barrel of a gun.

In regards to the topic, i don't think anyone's wallet needs to be insured. Avoiding single point of failure scenerios is the all the insurance anyone needs in that case.