Seriously, the amount of yak shaving I'm doing these days...
So I discover this CONFIG_USB_SERIAL_CONSOLE config option. I proceed to edit my .config, throw it in there and rebuild my kernel. (For the record, I hate building kernels). I'm about to reboot into said new kernel, and I thought I'd just double check the resulting config-2.3.36 file to check that CONFIG_USB_SERIAL_SERIAL did indeed get enabled. It's not there.
I spend a few minutes WTFing, and assume I've been an idiot and should have run a make oldconfig or some such thing after hand-editing my .config file. So I try that. The additional line vanishes again afterwards. More WTFing ensues.
I fire up make menuconfig, and go looking for the menu entry for the option. It's not there. The plot thickens.
I hand-edit drivers/usb/serial/Kconfig and change depends on USB_SERIAL=y to just depends on USB_SERIAL, and rerun make menuconfig, and lo and behold, the option appears!
So now I'm off rebuilding the kernel again.
Sigh.