Hello,
As other forum member said, this is impossible. You could trick the SRX into accepting Ethernet frames destined to several dst MAC addresses (with VRRP) but You cannot force SRX use different hw_addr in DHCP packet payload.
Assuming it works as You described, I have a feeling You bought the wrong service which is designed for L2 CPE, not L3.
Assuming it works as You described, Your options are:
1/ put SRX into transparent mode
2/ explore DHCP relay on SRX - the DHCP packets from SRX wil have same src MAC but different hw_addr in the packet payload. Not sure if Your ISP supports that.
HTH
Thx
Alex