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."

Roy...

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