I think you're on the right track. You've already called the recording what it is, evidence. The first question I would ask is, "what is it evidence of?" ie. provincial offence, or a criminal offence. Next is, does the 'evidence' on the recording device exist anywhere else, or is it the only source? If it is the only source for that particular evidence, then it is an exigent circumstance that it be seized immediately, to save it from being discarded or deleted.
Anything that is seized without a warrant is subject to a document called a report to a Justice, where an officer must declare before a Justice of the Peace that he seized something without a warrant, and thereby request permission to hold onto the item for a determined amount of time. If that is granted, the officer must write a search warrant to obtain the actual data from the device, whether it's a recording, or GPS data, etc.
The search warrant must answer three specific questions:
1) What is the alleged offence?
2) Where is the device?
3) Will searching the device produce evidence of the offence listed in #1?
If the warrant is granted, the device can be searched (forensically), and the evidence contained within can be used in court. From this point on, the evidence as well as the warrant itself can be cross-examined.
To answer your questions, yes this has been done before. It can produce some spectacular evidence, which in my experience has been the difference between a lengthy trial and a guilty plea. If the device contains evidence, and there is a risk that it could be lost if the officer lets you leave with it, there is a high probability that it's going to be seized, without your permission. But as mentioned, a warrant is required to view the evidence (as a result of a search).
Fun story: I seized a GPS unit from a vehicle where the driver was arrested for impaired driving. The GPS showed his speed to be 186.4km/h seconds prior to the traffic stop. It was a lot of paperwork, but I would rather spend a day writing a warrant, than spend the day being grilled on the stand in court.