For a third-party attack, the number of useful OS9 Mac slaves is limited by the connection capacity between the LAN and the Internet backbone. A single OS9 Mac with a good LAN connection could fill a T-1. Additional OS9 Macs on the same campus would not be able to add more bits per second to a third-party attack.
This in itself could be different form of attack, an "inside-out attack." Ongoing TCP connections to the corporate LAN (which require ACKs) would fail, and the incoming triggering packets would have ample bandwidth to continue to enter the campus network.
This could be a two-for-one deal for the attacker who could direct the outgoing packets towards a second victim.