Couldn't CloudFlare track a Tor user by tracking the tokens it gave to a particular user, then track them when their client used one of those tokens to validate?
If the tokens are never sent, only their blinded versions, it is pretty much guaranteed that the signature you get back was made without looking at the actual token.
I'm not sure, but it can be done with just CloudFlare changes; if the plugin is open source it should be fine. Maybe if Tor Browser integrates the plugin it should be fine too.