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to me, that sounds like an iteration issue.  so the IDL is calculated then the reflection.  i'm not sure it's nearly as complex as caustics to ask the renderer re-run the IDL at least once after the reflection.  or to allow for control over how many times it does iterate as a quality setting.  especially if  IDL doesn't affect specularity.

look a mirror can not bounce soft lighting. a mirror will bounce more sharp lighting since its a mirror. we can not use IDL soft bounced light for mirrors. we can fake it with an extra light.
maybe if we can in tell poser that insted of the mirror use a white diffuse shader. so we would have reflection but firefly would use a white diffuse for bounced light.