Hi Mark,
Please check "GW/ACL_MODE" parameter in your ECC environment. If it is enabled & no reginfo is maintained then EHSADMIN program will not be able to register in your ECC environment. First try to set "GW/ACL_MODE"=0 & then check the WWI program is able to register itself under transaction "SMGW" in ECC. Hope this helps.
Also you can refer "http://wiki.scn.sap.com/wiki/display/Security/Gateway+Access+Control+Lists" to get more detail.
Regards
Baldev Singh