Yes Aekeron, it was a DG834GT (Broadcom based) with the latest version of DGTeam's firmware. Also had the same result from official Netgear fw and UberGT with the FreeDG tweaks applied. Easynet's DSLAMs aren't Broadcom though, so... 

iptables -I POSTROUTING -t nat -o vlan1 -d 192.168.1.0/30 -j MASQUERADE
ip addr add 192.168.1.2/30 dev vlan1 brd +
The AM200 does indeed support ITU G.992.5 Annex M.
Make sure your modem is on a different subnet compared to the WRT.
I set my AM200 to 192.168.1.1, my WRT54GL is 192.168.2.1.
Go to administration --> scripts --> firewall. Copy and paste the commands below into the window, then click save.
Yes my thinking exactly.ExcellentI'd already had it confirmed by an end user, but it's nice to hear it direct from the horse's mouth. I noticed you said Cisco rather than Linksys... Linksys themselves told me it "may or may not work - plug it in and see" LOL If Cisco accept enquiries direct for Linksys products that might be a more worthwhile avenue in future?