I'm working on a browser add-on that has some novel usability ideas, and I'd like to test them out on some users. The add-on interacts with the user's password data and usage, and I'd like to collect some (non-identifying or exploitable) data. In particular, I'd like to keep track of how many passwords a user has, the average number of sites they're used on, the number of characters in each password, etc.
I realize that I'm treading on potentially shaky ground here. Where could I find users interested in testing such an add-on? How can I assure them that it's safe?
Other than open sourcing the code, which I'm already planning to do.