I'd like to be able to send an email to the nth person (or every n people) that submit a specific webform. I've tried utilizing rules for this, but I can't seem to expose webform submission data to rules (using submission id). Any thoughts on how this could work through rules? Any suggestions for alternatives?
Have you tried using Webform Rules ?
This should expose webform results to rules, then create a new rule with conditions "if submission ID is 100, 200 etc" then... action: Send email.
You would either need a way to find the nth of 100, or depending on how many submissions you are expecting, just check against a comma separated list, eg "100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200" etc