You can still use self-assessments, manager and peer reports on the work done, e.g. each quarter. Just explicitly don't use them for promotion. If they are used only for "whether this person brings more to the company than company spends on them", then it unlikely to become a metric to game around.
We're talking about leveling and compensation based mostly or entirely on years of experience; this implies they are based only a little or not at all on performance reviews.