I would like to request that there is an option in metacity to have the alt-tab application switcher display *all* windows from all workspaces in a global most-recently-used order.

I have about a dozen applications constantly running, on four different workspaces (virtual desktops). I switch between applications constantly. To show why the current method is not suitable (for me), let's look at an example. 

Assume I have app A on workspace #1 and app B on workspace #2. To switch from app A to app B I have two choices. First choice is using the mouse to either click on workspace #1 or on the window list entry of App A. As I prefer using the keyboard, this would mean that my hand has to leave the keyboard a lot. This is not economical and slows me down.

Second option is using Ctrl+F1 to switch to workspace #1 and (depending on whether app A has the focus or not) use Alt+Tab to switch to app A. If app A is hidden behind several other apps on workspace #1, this switch from B to A is taking me quite some keypresses (inefficient). But even if app A is the only one on workspace #1, I find pressing Ctrl+F1/F2/F3/F4 very uncomfortable.

Yes, I can assign "switch to workspace 1" to a more comfortable key combination. But then we have to consider the cases where I am not actually aware which workspace a window is on. In which case the only suitable option is to use the mouse (argh).

Having all windows of all workspaces in alt+tab is something I am quite used to. I've already looked into what needs to be changed and the general logic is quite simple. In display.c:meta_display_get_tab_list we would need to iterate through all workspaces (screen->workspaces) and in keybindings.c:process_tab_grab it would be necessary to call meta_workspace_activate_with_focus in case the selected window is on another workspace. Then of course there needs to be an option somewhere so this can be en/disabled by the user.

Requiring some more changes would be having a global mru list (i.e. not only per-workspace, but also for the screen object).