Thus this script. Feedback appreciated, of course.
Load it several times if you need several instant hotstrings.
Note: If you don't enter a hotstring, but do enter a replacement,
Edit April 2, 2018. Doesn't exit if you don't enter a hotstring, shows a message and returns to the GUI when you click OK.
Edit April 4, 2018. Does require AHK v1.1.28.00+ to be able to run (thanks, derz00)!
Edit April 13, 2018. Changed 'OldClip = Clipboard' to OldClip = %Clipboard%. Yikes...
EDIT: New version April 13, 2018. Now uses clipboard in the proper place, by using NewText := Hotstring(":X*R:"Hstring, "TextAdd").
The label TextAdd now (properly) sends the replacement string via the clipboard. Huge speed increase with large replacement texts.