we have the general question, what is the use case of updating a dialog from within the rendering pipeline? This seems pretty unusual to us.
In general you can use Core Messages to execute code in the main thread, see Core Messages Manual. This still works in R20.
In R20 there's ExecuteOnMainThread() in the maxon::JobInterface, see Jobs Manual. But I strongly suggest, that we first discuss, why you want to update a dialog from within the rendering pipeline. Otherwise you'd probably run into unexpected results with this approach.
I turned your thread into a question, see Q&A New Functionality.