You can’t open the Screen of any solution (“In maintenance”) from out of Solution Explorer.
Yet it is still possible to explore Screens of the “frozen” Solution. To do it,
1) open the solution you need to explore,
2) openUI Designer,
3) navigate to the Screen you want to open in the Configuration Explorer
4) open the screen you are interested in from the Configuration Explorer
Unfortunately it seems they removed this, because the “Open UI designer” button is disabled now.
Yes, this backdoor has been closed several releases ago.