I'm trying to do the same thing. Since workflows can only be created using constituent lists, once a constituent is already on the list, new gifts stop appearing (or at least that's what I've deduced based on some of my more recent gifts not appearing in my workflow).

Hi! Just curious, but are you not able to do this with Lists and Workflow Designer today? I was thinking you could create a list that looks for gifts given between a certain date range (ex: this month, or between certain dates) and then filter by the fundraiser. I am not positive that that is what you are looking for, but just wanted to check because I was thinking this was something you could accomplish today, but please let me know if I am missing something important!