Write to campaign member record custom fields

We know that Marketo can create a Campaign Member Record (CMR) in SFDC when the sync is turned on.  We know that they can write to the status field on the CMR. We know that Marketo can read the custom fields on the CMR.  It would be nice to be able to write to those custom fields as well.  Since Marketo already has read/write access to the CMR, it would only make sense to be able to write to custom fields.