Ordinal logistic regression and the relative risk


In binary and ordinal logistic regression we obtain odds ratios as
measures of effect. Because the odds ratio is only a good measure of
the relative risk when the outcome is rare, often it is desirable to
convert the odds ratio to a relative risk. In binary logistic
regression, the relative risk can be approximated by:

RR = OR / [(1-P0) + (P0*OR)]

Where,

RR = relative risk
OR = odds ratio from logistic regression
P0 = probability of outcome in the unexposed group


However, in ordinal logistic regression (let's assume proportional odds
ordinal regression), what the probability of the outcome in the
unexposed group would be is not evident. Furthermore, I have no idea
whether this approximation is even valid for ordinal outcomes.

Does anyone know how to estimate the relative risk from a proportional
odds ordinal logistic regression coefficient?

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