To fix that:
- Move the code from WikimediaEvents to ConfirmEdit.
- Use the AuthChangeFormFields hook to add a hidden field to the form which can be used to pass data from the client to the server. (That hook could also be used to add the logging module in a little more targeted manner than it is being done currently.)
- Log registration attempts to EventLogging on the backend (presumably in CaptchaPreAuthenticationProvider::testForAuthentication), with the input dynamics data if present. Also add in some useful fields like whether the registration was on the mobile site, and whether it was done via API.
- Maybe send some high-level info to Graphite to make it easy to track the ratio of human / bot signups (for some very simple approximation of who is a bot) - see T152219: Statistics on Captcha success/failure rate.