The Mechanical Interlock Attachment you describe (SQD #QO2DTI) is intended as a means of preventing two loads from being connected at the same time not as a transfer switch.
However if you use the interlock with (2) breaker retaining clips then it is UL Listed as a manual transfer switch in "dual power supply applications". But only when installed in a SQD #QO816L100 load center.
You can also buy it in kit form (SQD PK4DTIM4xx) for use in newer series main lug only loadcenters.
All of this information is from Square D's Digest #172 page 1-9.