How to send Ctrl+Alt+End to Remote Desktop?

Do you have the option of running AutoHotkey on the remote PC? You could run a script there which would trap an alternate hotkey of your choosing, and render it (there) as Ctrl+Alt+End.

Alternate method which opens the 'Ctrl-Alt-Delete' overlay manually:

Run > shell:::{2559a1f2-21d7-11d4-bdaf-00c04f60b9f0}

or

cmd > explorer.exe shell:::{2559a1f2-21d7-11d4-bdaf-00c04f60b9f0}