I have a /29 subnet assigned by my ISP, with an SonicWALL office router and a Cisco voice router hooked up to a 'net-rail' of sorts with untagged VLAN switch ports connecting my ISP router's output and the two firewalls.
I'd like to use this as a routed subnet so that all connections hit my office firewall and I can DNAT to our internal network.
Is there anything that can be done without significant changes on our ISPs router?
Once it has the appropriate DNAT statements, your Sonicwall will automatically Proxy ARP for the other addresses on that connected network to your ISP. You do not need to "route" them to the outside IP address of your firewall.