I happened to notice 'long running' as an option (and something called a 'progress indicator' when starting a microflow from an invoke-button on a form. That might answer the question more or less. But it isn't a property of the microflow, but an attribute of the calling event; that's something different, of course.
The microflow I had in mind when asking the question was started right from the menu, not from a button on a form. So maybe I only meant to ask "Is it possible somehow to specify 'long running' when starting a microflow from the menu. Either as a property of the microflow, or as a property of the menu-item".