Well, I should have put this here a few months ago, but I've been lazy to do so.
Anyway, here is Root Locus 0.3. It now supports discrete systems, and it lets you set a value for 'k' (the one of the regulator) manually. You can also ask it for the position of the poles in the current position. Everything is in the manual.
Link to hpcalc: http://www.hpcalc.org/details.php?id=7354
As of 2018-03-23, this project is also hosted on GitHub: