There's no way to really make a progress bar make sense, it's a luck-based mechanic.
Maybe one of those (slightly misleading) progressbars that have a dynamic speed that gets slower and slower the closer to the finish it gets? Just to indicate that it's working towards something
It'll be somewhat involved, but based on the difficulty vs the clients hashing speed you could say something probabilistic like "90% of the time, this window will be gone in xyz seconds from now"?
Yeah, I have to get the data for that though! I'm gonna add that to the list.
more, easier proof of works
and the law of large numbers will do the rest
That's multiplying the work the server has to do by a large number so it can show a nicer progress bar.
Seems very counter to the purpose.
So just like the windows copy dialog. Progress bar it is.