You can set up a direct disconnect REX with timer wiring by breaking the Ground (-) that goes to the maglock through the Normally Closed (NC) and Common (C) connections on the REX.
Run the Normally Open (NO) and Common (C) from the REX to the B and Negative (-) on the board and set up a REX rule. It breaks it manually and it is made known with the state change on the REX.
A System Event REX rule can then be set up to keep it open.
In System Events, create an On Rex Open Trigger, then create an Open Action.