Lease renewal (in the T1 stage) is facilitated via a unicast message from the DHCP client to the DHCP server. Why would unicast traffic from the client subnet not be routed to the server subnet? How would the original broadcast be relayed to the server subnet if the DHCPDiscover message from the client subnet is not being routed (relayed) to the server subnet? If no traffic is being routed between the subnets then how would the client obtain an ip address in the first place?
At the end of the day, the traffic from the DHCP client subnet is routed via the router (by way of a unicast message from the DHCP relay agent during the original DHCPDiscover stage and later during the T2 stage) and/or via unicast directly from the DHCP client during the T1 renewal stage.
As you stated, if the client reaches the T2 stage then it submits a broadcast renewal request, which the DHCP relay agent should relay (unicast) to the DHCP server.
Is this a theoretical question or are you actually having this problem?
To put another spin on my answer: When the client initiates a lease renewal in the T1 stage the renewal request is a unicast message directly from the client to the server (unicast is direct one-to-one communication), so the DHCP relay agent should not be involved at all. If the client reaches the T2 renewal stage then the renewal request is sent via a broadcast message and should be relayed by the DHCP relay agent (which is actually a unicast message from the DHCP relay agent).
If the T1 stage unicast message is not being routed from the client subnet to the server subnet then this is a routing/network issue, not a DHCP issue.