Hi Carlos --
If this is entirely opt-in by the user, for their own machine only, then I suspect you're right, that the overhead of an origin trial -- minting and distributing tokens, expecting them to be present in headers, collecting developer feedback, etc -- is unnecessary. Origin trials are particularly suited for developer-opt-in, where it makes sense for specific developers to enable a feature for their site alone. From your description, I don't think that individual PWA developers would need to add any extra support for the feature to their apps; you wouldn't be exposing any new APIs to the web, and there would be no reason not to offer this functionality for *all* PWAs installed on a user's machine.
More likely, what you would need is support for a Finch trial, wiring up the availability of this feature to a base::Feature setting, so that it can be controlled through the about:flags page, and disabled remotely in case of emergency. (It sounds like you might be doing this already, although I don't know what the relationship between Edge and Finch currently is.)
I hope that's helpful; I'm writing as an origin trial team member. Other folks (Fugu team or the metrics team, which runs the Finch infrastructure) may have their own opinions :)
Ian