Contributions of indirect research toward getting a post-doc
There's a thresholding effect for software. If you create a piece of software that has a huge number of downloads and has visible impact, that will count for something. However, with papers, you wouldn't need to have the same level of impact to have the same effect on interviewers. 

The problem really is that academics don't know how to evaluate the impact of software development, and would need help doing so. I imagine that if your potential post-doc advisor has used your software, you won't need to do much explaining. But if not, and your research credentials are otherwise weaker, it might be difficult to justify.