I must admit, I can’t see why you can’t mod them by checking various 4-digit blocks of their phone number in the title.
The false positives from putting 9132 or 2216 (or whatever, it is consistent, apart from some spacing, which can be got round by checking several blocks), in the subject line must be miniscule.
Still, I’ve been a mod on two sites, and one of them was a real battle of attrition, so I feel your pain. 