I don't understand this part. How do you select using Alt?
Typically with mouse enabled, you can select text with the mouse, but this doesn't go to the X clipboard. While with mouse disabled, text selected with the mouse goes to the X clipboard.
What I had meant was that when using vim configured with mouse=a, selecting with the mouse while holding down the Alt key would disable the terminal emulator's mouse reporting and allow the selection to be copied to the system clipboard.
But actually this depends entirely on the terminal emulator. The Alt/Option key works with the default preferences for iTerm2 on MacOS, but with GNOME terminal I reproduced the same behavior using Shift instead.
Typically with mouse enabled, you can select text with the mouse, but this doesn't go to the X clipboard. While with mouse disabled, text selected with the mouse goes to the X clipboard.