I tried to specify a different dhcp-lease-time value in the latest version of opnsense. (Found referenced here https://forum.opnsense.org/index.php?topic=20323.0)
This is what I put in the Option Modifiers on the WAN interface under Advance.
supersede dhcp-lease-time 86400
Enabled and disabled the WAN interface to attempt to pick up the value. When I cat the file /var/db/dhclient.leases.re1 I get (re1 is the WAN interface)
...
option dhcp-lease-time 3600;
option dhcp-message-type 5;
...
I expected by override value to be specified?