From what I know about the host: device, it isn't too good.
It seems to have been introduced with the PCSX2 v0.9.8 official release, and older versions that were downloadable from their main download page lack that feature totally.
It's also not enabled by default, which meant that the user has to enable it by checking the "Enable host filesystem" option under the System drop-down menu option.
The device is host:, and doesn't seem to work under certain circumstances that I don't remember right now (But it's related to accessing the device from an usual location... was it the IOP?).
And it's also incomplete; The stat() function is not implemented.
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit