Problems with minicom over USB

Embedded systems (especially bootloaders) need serial connections but many laptops and modern desktops simply do away with the serial port altogether. Up until now, I've kept my previous desktop machine around (which happens to also have a parallel port so acts as the home network print server because I don't have a USB printer either) and then connect to minicom over SSH.
Now, yes, I can configure networking over USB once the device is installed but to actually fix installation problems, I need serial on my other machines. I've got a USB to serial converter, turns out to be a Prolific 2303 chipset (it's the basic one with a cable from Maplins) which is quite well supported.

Now the weird stuff.

My powerpc laptop sees the Prolific chipset and configures a USB device that, initially, minicom complains about - but if I set the port from 'dmesg' (/dev/ttyUSB0) in minicom -s, it works. Well, almost. I can see the output of the device but no keyboard strokes are passed on to the board which means it's almost workable whilst still being completely useless.
My problematic HP laptop accepts the Prolific chipset but does not configure a USB device node. Without a device node, minicom just fails.
Bah.
This HP laptop is still causing me grief. USB seems to fail after hibernation and only works at all if irqfixup is added to the kernel boot parameters. With irqfixup, the laptop randomly hangs. I almost need to setup parallel configs in grub of "run USB but don't hibernate" and "run for more than 1 hour" as mutually exclusive. Oh, and the external monitor connection used for presentations is only seen if the projector is connected during a reboot of the HP laptop.
Bah x 3.