I managed to write down what I conceptually want to create. You can read the description here.

Notice the difference with the Ripple, in the use of a shared Bitcoin wallet to back IOUs.

I don't know whether this can be implemented on the Ripple network. I suspect it is not possible: if I am correct, there are only a limited number of message types in the Ripple protocol, and some of the things I want to do probably don't fit in these message types.