I haven't tried to harrier roll on the sim. I'm pretty new to it in real life. On my 35% extra 260 I've found that I can harrier roll using just the elevator. I use only about half the throw that I use for waterfalls and stuff.

Here's how I do it :

1) get in a harrier
2) add full aileron deflection and about 2 clicks of throttle (I don't use a rachet and couldn't get the throttle precision I needed with one)
3) full down, center, full up, center, full down, center....

At first the plane had a tendency to work the nose up into a hover. I fixed this by decreasing my elevator throw for harrier mode until it stopped.

I've done harrier point rolls and harrier slow rolls using both the rudder and elevator. I've never tried normal harrier rolls with just the rudder. Now I have something to do this weekend!