Pricing a perpetual subordinated bond when there is risk - using an example

Orga

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In 2011 the then Minister for Finance indicated that he would write down the value of the PIBS which BoI inherited from Bristol & West in the UK and which is a perpetual subordinated bond. After a bit of a battle, he relented. The media reportage referred to BoI meeting its liquidity needs and there being no need for the application of the Subordinated Liabilities Order.

So, I'm wondering if anyone would hazard an approach at how to price/value such a bond. For example, in "normal" circumstances you can calculate a bond's price based on the present value of all future payments and the regularity of the payments makes the calc pretty straightforward. But, what this approach doesn't involve is the risk, which the above scenario contains. Would you, for example, apportion risk to two separate calcs, say 80:20 and sum the resulting values. Or does anyone use anything different?