HOSTFS is pretty much like any other module/device combination under RISC OS. The module translates the FileSwitch calls into something more generic and exchanges commands/data with the 'device' via memory mapped registers. The device happens to use Win32 calls to access files on the host filing system, it could do just about anything. As Peter says, a HAL wouldn't make this any easier.
What a HAL _might_ do in the future is avoid me having to write emulation of specific chips that RISC OS expects to see and are needed for RISC OS to boot, load modules from podules etc. I.e. VA would become a platform in its own right rather than pretending to be an A5000/RiscPC to please RISC OS.
Even so, it is probable that the RISC OS 5 rom is custom compiled for each platform so VA would still need to emulate a particular platform unless it got its own custom build.