I've been following this thread and have a similar issue which is rather puzzling. I can control the machine with VNC, but would like to use a workstation (on an external network) to control the server via the managment console.
More specifically, I have a netgear router in front of the ISA server and the control workstation is external to that router.
So far, I have reverse mapped a port on the router to the external interface on isa2004. And I have added the external interface network range to the allowed management computers list in the system policy.
However, this arraingement doesnt work. I either get a access denied or a RPC server unavailable error.
Well I fixed it.
It turns out I had everything configured ok in terms of ports forwarded on the router, published rules, and a system policy correctly listing remote administration computers.
The errors were from an authentication problem. Given the machine is in a test lab, I neglected to give the administrator account a password. While ISA didn't care any about that, it mattered for remote management authentication, because logons using administrator (without a password) fail.