Hi Sean,
Yes, I believe you are correct -- that property on course offerings doesn't appear to be exposed through the API, so I don't think it's settable or modifiable through the API. I would encourage you to log this deficiency as a defect through your organization's Approved Support Contact, or your Account Manager. Although technically speaking it's not a bug (in that it's "working as designed"), in my opinion this is arguably classable as a defect for functional deficiency (in that, if the API allows you to "create course offerings", it should allow you to do so in a reasonably full and complete manner").