My post in the Managing Up series offers quick tips for how to do a quick audience needs analysis when you don’t have good info from product managers: “Who Are You? Who? Who?”
These tips work for assignments to document a small tool or app when you receive only a broad-brush statement about the target audience for the software.
My basic guess? For these kinds of (typically free) tools and apps, there’s something out there that’s similar. You can pretty easily find out who uses such software and identify the nature of common problems.