@hasherati first of all, I would like to apologize for the inaccurate information I shared earlier. The feature will be included in 8.5.2 is RUA - DHCP reservation based on user ID, which allows you to reserve an IP per RUA - Username defined in RUA - Local User Accounts. It should achieve the objective but in a different approach.
NOTE:
RUA = Remote User Access (L2TP IPsec, PPTP, OpenVPN).
Our team is working on it to make sure it will be ready as part of firmware 8.5.2 standard feature. Stay tuned.