Not sure what else to add here. BadukPop is a very popular app for both games and tsumegos, and it would be great to have it in the list.
Does badukpop have kyu/dan ranks or just a rating?
Both! Rank on BadukPop is a bit more sophisticated thought. There are two separate ratings - games and tsumegos. You receive a new rank once you reach a specific value in both (!) ratings. E.g., to reach a 7k you need to have 3700 points in both games and puzzles.
You could try adding it as a feature request on github: GitHub - online-go/online-go.com: Source code for the Online-Go.com web interface