Simon> it up to my G4, plugged the other end into the modem port on the 5500. I get
Simon> a connection between the machines (typing on one outputs on the other using
Simon> MacOS ZTerm on both machines) then reboot the 5500 using "cmd-opt-O-F".
Simon> Nothing much happens on the Zterm on the G4 (no 0 > prompt). I have setup
Simon> zterm with 38400bps, 8 bits, no par, 1 stop, no handshake shake, kermit etc

Sounds like everything you did perfect. You say
'Nothing much happens' meaning some garbage came up, didn't it ?
 
Instead, if really 'nothing happens at all',
the only thing I have in my mind is, sounds ridiculous (even to me) but,
the possibility that Zterm requirement and OF one is different.

  I am experienced StarMax User :-), unfortunately I have only 4xxx
and 3xxx. With my practice, I needed always the special cable for
serial connection for OF interaction.  Mac(StarMax) side, RTS, pin 4
if D-sub 25 notation, did not come up.  So I am using following cable
to get CTS enabled on Terminal side.

       Mac cable         Terminal computer side
    1  FG
    2  SD  TXD   ---->   3  RXD
    3  RD  RXD  <----    2  TXD
    4  RS  RTS    +-->   5  CTS
    5  CS  CTS <--+---   4  RTS 
    6  DR  DSR          20 (DTR)
    7  SG                7 (GND)
                         5 (GND)
    20 ER  DTE <--       6 (DTR)

---
(Makoto Fujiwara)
http://www.ki.nu/software/NetBSD/iBook2/