IPCop Packet Mangling

I've found myself in a pickle replacing an old firewall for a client this afternoon. I'm configuring their new IPCop firewall (1.4.21), Zerina OpenVPN addon is installed.

What I need to do: There are three network interfaces, currently set up as red (WAN), green (LAN, 192.168.20.0/24) and orange (remote network 10.1.20.0/24). The orange interface is a direct fiber link to another organization.

Simple description: Traffic and networks appear to be properly configured at this point, but I have many (150+) specific IPs on the LAN which, when accessing the resources on the 10.1.20.x network, need to be mangled to appear to be coming from the 10.1.20.0/24 network (and return traffic properly delivered). The routing on the far side was configured earlier and should be fine, but I need to redirect any packets coming across destined for those IPs to end up at their proper destination.

The addressing is fixed and predictable (ie. 192.168.20.125 -> 10.1.20.125). I need to insert whatever rules I have into the IPCop ruleset through /etc/rc.local I know, I'm just not sure about how I should structure this. There's CUSTOMOUTPUT and CUSTOMINPUT targets, both which currently just consist of the single rule redirecting packets to the OVPNOUTPUT/OVPNINPUT targets, so I'm guessing I should insert a rule matching outbound packets destined for the 10.1.20.x network and redirecting to a new target (maybe called TO-ORANGE) and a rule at the top of CUSTOMINPUT which redirects to a FROM-ORANGE target. Under those targets, I would have rules which do the IP matching and mangling.

Am I approaching this right? If so, I'm not very familiar with mangle, and would appreciate seeing examples of how to write that source-IP rewrite. If not, how would you suggest doing this?

TIA!

edit: I notice additionally that the nat table has CUSTOMPREROUTING and CUSTOMPOSTROUTING targets, I guess I could alternatively post the rules in there....