To answer my own question, I had to create a Virtual IP (IP Alias) with the single static IP address that the DNS record points to. Then, under VPN -> IPsec -> Mobile Clients -> Edit Phase 1, under 'Interface' the Virtual IP created is given as an option.
I also changed the way the pfSense firewall/router obtains its IP address. The WAN interface now has a static private IP address (192.168.2.1) which is seen by my ISP's gateway device, along with the Virtual IP. (The gateway device is, of course, set properly so that traffic to pfSense isn't filtered or blocked).
So now my IPsec VPN works with one of the static IPs, and traffic from the computers behind pfSense is seen as coming from the DHCP address assigned by my ISP, as I need it to.