I have done it where I have two plants and a walled in area for "guests" or even other sims that live in the household. They can live there or not as long as you lock they in with the plants or wall them in somehow. It might work better with clubs really but never used them for this process before. I plan on doing that next though. The sim has to eat twice of course.... I do think it might fail now and then because a sim is stubborn. You also have to take into account that a cowplant will die if it's produced the cake and hasn't been fed for 12 hours.
I currently have a save where I have a plan to use cow plants eventually. I play slow so it might be awhile. The main character is an evil scientist and he plans on using his test subjects (rich married townie guys he cloned and replaced in their households.. why? why not?) as cowplant food for awhile. I plan on having out a variety of aura objects during the testing process in the hopes of getting a good variety of mood drinks that are produced with the first digestion. If and when they die in the process it will be duly noted in the science logs.. which will in turn of course be a boon to the knowledge base of simology in general.. right? Of course right. There has to be a reason for all this doesn't there?
Also.... evil sims get a kick out of being around others that have bad or uncomfortable moods... even those that they love. It just tickles them. So your sim can always check on the progress of the victims now and then and get happy from that if they are uncomfortable, tense, embarrassed or angry.
Take into account that your evil sim also gets just as sad as others when anyone they like (even barely) or love dies. So if you want them in a dance on the grave kind of mood it's best to make enemies with the victims first. So if you are playing a black widow be prepared for her 3 day sadness after. I think if you tank the relationship you can avoid it but not sure.. it's been awhile.
