Well, after many failed attempts, here I am on my problematic HP laptop with a USB key mounted, a balloon3 board booted under an Emdebian root filesystem and visible over USB-to-serial using /dev/ttyUSB0 via screen.
Thanks to Daniel's tip about screen /dev/ttyUSB0 115200, my USB to Serial converter (Maplins with a Prolific PL2303 chipset) is working on my problematic HP laptop.
The proviso is that I have to reboot to enable all this stuff - allow the laptop to hibernate and all this USB stuff just disappears.
The only remaining problem is the wireless - I haven't decided yet whether to patch the current stock Debian kernel (I've had enough problems without getting into more kernel issues) - I may simply buy a replacement external wireless unit or just not use wireless until the b43 driver is capable of working with the Broadcom Corporation BCM94311MCG wlan mini-PCI (rev 02).
The magic runes for enabling USB are to add some more options to the grub boot list (on the 'kernel' line):
noapic nolapic noirqfixup
minicom still doesn't like serial over USB (on this laptop, it adds an extra line feed and then refuses to pass on any keyboard entry to the device in a similar manner to the powerpc laptop) but screen works nicely. Thanks Daniel.
With noirqfixup added, I can also plug in other USB things like a USB mouse and actually see them work. This is all good news because apart from adding these mysteriousrunes to Grub, this is a completely standard Debian box running Sid and with no particular configuration changes or tweaks in the system setup.
This means that I can now get on with uploading the emdebian-tools package split to Debian unstable, adapting the Emdebian root filesystem for balloon3 to support USB networking by default and suggesting screen as an alternative to minicom in the control information for the new emdebian-rootfs package. (The package split is now in Emdebian SVN - main trunk - if you are interested.) I'm also downgrading my old G4 iBook to running Lenny instead of sid and it will now stay running Lenny as Debian stable when Lenny is released. It just makes it easier to not have to keep downloading massive updates on the brief occasions that I now use the iBook.
Now if I can just work out why hibernation forgets 'noirqfixup', I'll have a usable HP laptop....
Not bad for a machine that I once considered as a complete non-starter.