In GNOME bug 120204, someone suggests being able to pick up all the windows of an application and move them to another workspace, and also to minimise them, close them, etc. This could be done by holding down some kind of modifier while the function is being selected; this may violate the principle of least astonishment, in case someone made all their windows disappear without meaning to, merely by holding down Shift or whatever.
On the other hand, someone (who apparently used Vietnamese localisation) suggested using dialogues, with this as a possible UI:
I think this is perhaps over-baroque. I’m not sure this feature is useful enough for the downsides of either method to be worthwhile.
Photo © Jay Gooby, cc-by.
If you ever held down Alt+SysRq and got 27 print screen dialogs as a result, you’d miss the ability of selecting a bunch of windows and closing them all at once.
IceWM had a window list dialog (bound to Super+Esc) that let you select multiple windows and perform an operation on them (close, minimize, move to workspace N). Here’s a screenshot: http://gedmin.as/icewm/winlist.png . It’s a typical multi-selection box with a context menu with window operations, and some keyboard shortcuts (IIRC pressing Del would close the selected windows).
Sometimes I think Metacity could use something like that.