Default risk can only be perceived externally, as it is an event that is most likely only happening once (e.g. pirateat40 paid consistently every last Bitcent until it all went POOF and then he consistently paid 0 Bitcents any more). Also Web of Trust ratings won't help much (again: pirateat40 had a flawless and highly rated WOT account). There might be some service option similar to rating agencies that might release their opinions in a format that oyu can plug it into your Ripple client automatically, but still knowing the real risk is not something that can be really calculated.
Back before banks discovered they could just dump their losses on the taxpayers and depositors, they developed a series of techniques for estimating default risk that work reasonably well, and people who want to use Ripple successfully would need to use similar techniques in order to estimate the amount of trust they should assign.That means borrowers should disclose their balance sheets to creditors, develop rational methods for assigning value to their assets (reserves), and pay attention to debt to asset ratios as well as debt service to income ratios.
None of those things happen within Ripple; they have to be done manually, out of band.
You could, of course, just assign trust based on other factors without taking any of the above into account but that would be exactly like throwing money at pirateat40.
