First, change your IPsec tunnel network to all-nets on main office's side (on main DFL it will be local network, on branch - remote).
On "main" DFL make rule NAT ipsec/ipsec_remote_net wan/all-nets http
On "branch" DFL
- change in rule Allow lan/lannet ipsec/ipsec_remote_net destination network to all-nets
- add new routing table (ex, thru_ipsec) with just one route ipsec all-nets 100
- add new routing rule lan/lannet wan/all-nets, service http, forward thru_ipsec, return main
Last rule will change route to ipsec interface for all HTTP traffic from LAN.