I've come to realise that footprints are the key to getting links that you theoretically shouldn't be getting. A new site shouldn't be getting high PR do-follow links but with good footprints anything is possible.
I have found that I had some ridiculously complex footprints, not out of desire for them, but out of necessity. However I always strive for simplicity, and if I have a simple footprint which gives me exactly what I want, then I use it. In fact as I said the most simple footprint is what I aim for. You might start with a simple "Powered by X" example but as you say that is too broad. You analyse the page and then add something else which will focus and drill down even more; you keep going until you have reached your target, i.e. almost every page that SB/HREF harvests contains the comment box you are looking for.
What makes a good footprint? I define it much like crazyflx, and it's funny that we both had never talked about it really, but came to the same conclusion. I am looking for the following:
1. VOLUME. I must get large amounts of results. It is no good being so specific and picky that you get near zero results. That's the tug of war of a footprint: volume is pulling on one end, telling you to be broad and get the most results, while accuracy is pulling you on the other end, telling you to be more specific and add more criteria, words, special operators etc. The footprint you find MUST allow you to get large volumes, with a good keyword list (1 million+) a good proportion of the platform in question.
2. ACCURACY. It is no good getting millions of results when 95% of them are useless to you. You may as well just use no footprint and harvest using just a shitload of keywords, because your list will be useless to you if each page is not what you need. Your footprint must be accurate and provide 95-100% of pages with the criteria you need.
The best footprints combine volume and accuracy magically. I love it when I find a new one.