Message tracking does indeed track messages delivered to CRs, because it records deliveries to all GAL objects (so long as you've turned it on).
As CRs are external objects, it is sufficient to check just the tracking log on the server that hosts the IMS.
However, there isn't a specific event that signals delivery to a CR, so you'd need to step through each tracking log entry, and for every list of recipients in it, step through each recipient looking for CRs (this is easier if you have all your CRs in a CR-specific container).
A script to do this can run each day to scan the previous day's tracking log, and record the results. Do this for a period of a month or two, and you'll have a record of every time a CR is emailed. It's then a matter of processing the data to discover which CRs you have that haven't been emailed in the monitoring period.