I love answering my own questions..(apparently)
If you set up a firewall rule on the packets in question and check "allow fragmentation" this
appears to trigger virtual frag on the VPN tunnel. It didn't work at first, but the badly behaved
app was just behavng more-badly than usual. Working now.
From: Keith Redfield [mailto:kredfield at airsurfwireless dot com]
Sent: Fri 1/28/2005 7:56 AM
To: m0n0wall at lists dot m0n0 dot ch
Subject: [m0n0wall] Force MTU?
Is there any way to force MTU/fragmentation across a p2p VPN? I've got a badly behaved application
that insists on 1500bytes and sets DNF.