I think in general it´ll be VERY hard to find anyone to offer you a comprehensive vehicle insurance, that´d be valid world-wide. Insurance companies just simply lack data from outside the continents that they operate in, so they are unable to do their own risk-assessment. Gathering such data for the handful of overlanders, who might be interested in this (many of whom might still say ´no´ after hearing, what it would cost!), it´s just easier for them to say they don´t do it.
Also thinking about how erratic traffic can be in many countries in, say Africa, or Asia, and keeping in mind also, that many locals may be riding or driving fully uninsured (so you will get zero compensation from them, if they crash into you) I can´t say I really blame them!
It is funny, though, that I can easily get comprehensive insurance, also covering theft, for Green Card countries (that includes places like Paris, London or Amsterdam), but if I leave the Green Card countries, then it won´t be valid at all. I think the risk for theft is by far highest in those big cities in western Europe.
But it is more or less 5 years since I did some research on that, so things could have changed. You could try asking these: