It times out for any period longer than a few days. Easiest option seems to be to not make it available there, since this metric does not make sense there in the same way as it makes sense in translatewiki.net.
QA plan
When deploying configuration change, check on Special:TranslationStats and on Special:ApiSandbox#action=translationstats that registrations is not available.
Outcome
It is not possible to produce statistics about registrations using Special:TranslationStats or translationstats Action Api.