I used my HR as a filter last time I brewed; worked OK until it clogged. I stuffed a 5-gallon paint-strainer bag in the HR (used to use these for partial-mash) and as soon as the hop-sludge (pellet hops) built up a little, it forced the strainer bag to the top of the HR and plugged it solid!
Next time I'm going to use 3 or 4 stainless scrubbies in the strainer bag so it'll hold its form and not get compacted, and I'll also throttle back a little on the valve at the boil kettle to limit the flow through the HR.
Also; no matter what you use as a filter media in the HR, as soon as the boiling wort hits it, it's getting sanitized. IMO, there's no need to worry about pre-sanitizing - just be sure all is nice and clean before putting to use.