We can definitely store the "originalValue" off to the side.

But, given that our API is defined as only operating with mg/dL data I do not believe this is an issue for data provenance.  The minute we receive this data, that's when our duties for provenance begins.  The integration of an external system with us should take into account the fact that we only store data in mg/dL when doing the integration.  If they do not like our algorithm, they can convert ahead of time.  If they choose not to convert, they are agreeing with our algorithm.  As long as we believe in our algorithm, we are doing nothing wrong.