I think you vastly overestimate the work to be done to get something useful, and underestimate the work already done to get that far.
Implementing the low level USB driver to connect the stack to the hardware is certainly (if the docs are there - and since there is a Linux running, it should not be overly difficult) doable. Having USB running automatically enables keyboard, mouse and mass storage. The video is certainly good enough for basic RISC OS usage. Sound support is completely non-essential, as many A9 and Omega users will happily(?) confirm.
There are obviously a lot of things to be done from the "nice-to-have" department. Having a driver to connect the SD/MMC interface to the SCSISwitcher would enable non-USB mass storage. Using the OpenGL accelerator for video would be nice. And implementing a driver for a USB network device would be very good.
But compared to the work that was apparently needed to port RISC OS to other hardware in the past (initial IYONIX work, A9, RiscStation, Mico, Omega), I am pleasantly surprised what two dedicated individuals can achieve in a comparatively short timeframe.
One of the most important things for the platform (IMHO) is to build up porting knowledge and a driver repository to make RISC OS more easily portable. With the hopefully soon-to-be-available ARM-powered Netbooks, we might have a nice range of RISC OS-capable hardware soon.