My understanding of ISA is that it's just a proxying firewall. Can it really do routing as well? I know it can handle VPNs etc but I thought it wouldn't be able to do stuff like load balance 2 or 3 WANs hence the need for RouterOS.
Please update me.
I haven't used ISA recently, but ISA was never designed to be a Cisco-beater. It's primary use is for internet access control and security, so in effect it is just a caching proxy. It uses the underlying Windows Server RRAS to provide routing, VPNs, NAT. I don't believe it can load balance WANs as effectively as RouterOS can.
What you are trying to achieve should be perfectly possible, since to the ISA, the RB1000 will just be an internet connection.