Well for $20 (US) a year, This firmware might run on your clients router and
allow you to bring up an IPSEC tunnel to your m0n0wall:
"Talisman - VPN: The VPN build includes a PPTP and IPSec server in addition
to the core feature sets. This version is designed for high-security
installations where the router itself is a highly secure endpoint and
gateway to the internal network."