This SWS actions works for any envelope and it always obeys current grid
(but it will still snap to grid) so you can at least free hand in tempo envelope in other resolution other than quarter notes:
Code:
SWS/BR: Freehand draw envelope while snapping points to left side grid line (perform until shortcut released)
For more information on tempo stuff in SWS go read documentation in this thread:
https://forum.cockos.com/showthread.php?t=118061