In batch mode while entering gifts, there should be a way to tell (possibly by adding a column) that will tell a data entry person that this gift is their final pledge installment. We like to send a different letter to members once they fulfill their pledge.
Interesting idea - not a programmer but don't think it would be possible while in uncommitted batch as software does not yet know if payment is for whole balance or just part.
However, I can see if it's the final amount when I go to apply it to the pledge. At that point I can see/confirm payment amount and the display shows me pledge amount and balance. You might be able to pull it from there. Just a thought