It would totally depend on what is going on in the room at the time. I would only EVER put an 18month old that just soiled themselves on the potty AFTER being cleaned. I wouldn't just strip the diaper off and plop them on making a mess.

If it isn't too chaotic and the child is used to going on the potty at home I don't see too much harm in doing a partial diaper change, plopping her on the potty for a minute then saying all done, you already pooped in your diaper, no more poop for the potty and finish the diaper change.

THAT SAID...not sure I would put that extra effort in to a child that is leaving the program. So again, it would totally depend on how much time and energy you have. But, keep in mind it might save you time and stress by putting her on to satisfy her apparent need to go on it after soiling herself (and quiet her). As much as she is young and may not be ready she has clued in enough to say potty after having a poop...and that is indeed a sign of becoming aware and with the right approach it could very well lead to an early trained child. BUT...since the child will soon be gone that doesn't benefit you!