Just FYI: the issue does not apply to Create. On Create, since there is no existing Content Type field on the record being created, we were not able to do the above. But Content Type is a querystring parameter on create (or, actually, the GUID of the content type). So we are using that as our filter value.
Which makes me think I might be able to do the same thing for Read and Update by changing the qListViews on the default.aspx and elsewhere to include the content type as a querystring parameter. I will try that and see how it goes.