Will the consent to send SMS field work on unsubscribed records?

While the integration can't normally update the fields on unsubscribed members via field mappings, consent to snd SMS allows updating in that fashion. 

When updating the consent to send SMS value, please keep in mind the following:

• Processing time: Updates can take up to 30 minutes to reflect in Campaign Monitor.
• Field creation: There is not a standard field in Salesforce for consent to send SMS, so you will need to create one.
• Mapping: You can use Field Mappings or via the Import Wizard, which has its own field mappings.

How Updates Are Processed
Even if the corresponding list membership record is Unsubscribed;

• Changes will be passed when the record is edited if Field Mappings are used.
• If using the Data Wizard, updates will be processed when the wizard runs, even if the corresponding list membership record is Unsubscribed.