Typical pigging procedures shouldn't allow leaving a pig barrel isolated, full of liquids and under pressure so this should be a pretty rare incident. Concur that it's not a bad idea to have a thermal relief and that this should be tied back into the pipeline to prevent atmospheric discharges. A small 3/4" TRV is pretty cheap even with a little bit of piping.
To be honest, I don't think I've seen a pig trap with a thermal relief on it before and none of the standard pig trap drawings that I've seen used have shown this detail. Doesn't mean it's right though. Perhaps it hasn't been an issue as with the climate around here, the contents would probably tend to cool rather than heat up.
I believe the Canadian Association of Petroleum Producers is currently working on a guidance document around pigging and pigging procedures. No idea when it might be published though.
I took a quick look at the Norsok Specifications (the Process Systems document) as it has design requirements for pig traps and while it addresses interlocks, vents, drains, pig detectors, etc, it doesn't talk about relief so I'm guessing they are relying on the pigging procedures as well. I also looked up the Oliver Compact Pigging Systems (CPS), a complete packaged solution, and it isn't equipped with thermal relief either.
On balance, I think most people rely on procedures.