If you can get these sites to link back to you with their anchor text then I think you should be good to go. However, you should make sure not to link out to these sites as they will not be a relevant. I dont think getting to many links will hurt you as long as you dont link out to them.
If you do 100 irrelevant links and do 10 relevant links - both have same importance means do relevant links only as it has high weightage compate to irrelevant!
I don't know exactly,but it seems many people are doing it on DP forum including me. You will find most of the signatures on this forum have irrelevant sites!