I wouldn't risk it if I were you!
I expect government bonds (particularly in developed Western economies) are pretty close to risk-free in the currency in which they're denominated: I'm not sure of the appetite of any government to default, as opposed to simply printing money. At the moment, "printing money"/quantitative easing seem to be the flavour of the day. Of course, governments are currently all too happy to encourage inflation: the picture may be different in the (not-too-distant) future when the quantitative easing has caught up with, and perhaps overtaken, deflationary influences. The inflation introduced will then make it easy enough to meet the fixed interest government bond liabilities in any case. (On the other hand, printing money to pay off inflation-linked bonds could have the potential to be more inflationary...)
In general, government bonds will be held as a match for fixed liabilities in that currency. To swap into silver/gold would effectively introduce currency mismatching. You would only do this to the extent that you want to reduce your matching anyway, and so could go for a variety of different investments, of which gold and silver are but two. So I would have thought that the decision to go into such commodities is a separate one from that to decrease currency matching.
More views welcome!