I haven't tried it yet, but it should work on the A320 FMS/FCU:
f you want to intercept an arc that is already published and available in the FMS, but taking it downstream of the IAF, get the IAF as "FROM" waypoint in the FMS while steering in HDG mode and join at the desired place with NAV blue (HDG selector pushed while in HDG mode away from FMS planned track), it should intercept the track, shouldn't it?
As I said, haven't tried it, anyone who did?