If you go to the league breakdown, you'll see there are two tables, one for average and one for replacement.
You'd have to pull that data from the rankings themselves, I guess.
I'm not sure what valued you'd get from comparing against different RW slots (#1 RW, #3 RW, etc) though... either way you want a single "reference point" against which to compare a player, regardless of which RW spot they fill.
... That's the downside with z-scores.
Hah, funny how perspective works -- I'd argue that's the advantage with z-scores. If a player is soooo bad at filling up a given bucket, even if they're filling a few other buckets, it should be a major hit to their value. Downplaying those "knocks" has a massive cumulative negative effect on the performance of a team.