In theory, could this be used to make Keen not bounce at all? And if so, how about not moving, either? (Although it is my understanding that leaving the screen through the bottom or sides is how it triggers the "You didn't make it past" dialogue...)
Yes; both the x and y values of Keen's death can be changed, but if Keen doesn't exit the screen; he'll be stuck. However, the game actually counts $64 ticks after this happens before moving on to this code, therefore, I can patch it so that when Keen dies the dialouge box pops up whenever you like; Keen doesn't need to exit the screen and the pause can be made as long as you want. (There will be however, no death sound.)
#Didn't make it past appears instantly when Keen is killed:
%patch $CC8E $1A $00 #Sound played (In this case none)
%patch $CC7E $BB $3A
%patch $CC4E $BB $3A
%patch $CC6C $BB $3A