That would work to get the closure to have that isolation, and therefore it would make the function passed as closure run with that isolation (since it’s synchronous).
However, the function passed to that parameter wouldn’t know that it was running with that isolation. If it needed the isolation, it would have to reassert that with something like assumeIsolated. A better tool for that is the upcoming proposal for @isolated(any) function types. But that proposal will combine well with this one — instead of passing in the actor separately, you’ll set the isolation in the closure (e.g. with the features in this proposal) and then pass it off as an @isolated(any) function. Any call to that function will then know it has to run it on the right actor, and the function will still internally know that it’s isolated.