Hmmm.... interesting suggestion. i'm not mechanically inclined, but I like to brainstorm, so here goes! 
Since a baby swing goes back & forth, maybe the motor could be hacked in some way to make a rocking chair rock (back/forth motion) or almost be used as shoulders for a prop that then moved bent upper arms forward & back. Depending on how the motor was placed, maybe you could hack it to almost give a prop arm a forward thrusting motion like knife-stabbing, know what I mean? If the swing motor was set on its fastest position, it could make person/ghost prop look like they were shaking/rocking back & forth. Could be really good for a mental patient prop in a straight jacket.
If you had some trees or built a scaffolding of some kind, maybe the back & forth motion could be used to sway some rope nooses with some skeletons in them.
Or if you attached the motor hanging down from a tree branch, you could hang sheets, netting, plants/vines/leaves, lanterns, etc to give a sense of movement to a graveyard or scene outside.
Or if the motor was on its side, maybe you could attach a faux doorframe and the motor would move it in an opening & closing manner, or make it into a creaky gate or something like that.
No idea if any of these things will work or suit you, but there you go!
Since a baby swing goes back & forth, maybe the motor could be hacked in some way to make a rocking chair rock (back/forth motion) or almost be used as shoulders for a prop that then moved bent upper arms forward & back. Depending on how the motor was placed, maybe you could hack it to almost give a prop arm a forward thrusting motion like knife-stabbing, know what I mean? If the swing motor was set on its fastest position, it could make person/ghost prop look like they were shaking/rocking back & forth. Could be really good for a mental patient prop in a straight jacket.
If you had some trees or built a scaffolding of some kind, maybe the back & forth motion could be used to sway some rope nooses with some skeletons in them.
Or if you attached the motor hanging down from a tree branch, you could hang sheets, netting, plants/vines/leaves, lanterns, etc to give a sense of movement to a graveyard or scene outside.
Or if the motor was on its side, maybe you could attach a faux doorframe and the motor would move it in an opening & closing manner, or make it into a creaky gate or something like that.
No idea if any of these things will work or suit you, but there you go!
