I still think the Harrier isn't the ideal aircraft for SEAD'ing. DEAD, yes why not? Along with A-10s and F/A-18s? (IMHO DEAD is more fun :D)
In terms of IR guided SAMs and AAA (SHORAD), you can do a pretty good job with the A-10C, it has MWS in case someone has the line of sight to shoot at you. A-10C has its own limitations too, speed, altitude recovery, ECM etc.. But it has an excellent combat range and payload. Which isn't really the case with the Harrier.
The Harrier however, can land anywhere. Being the aircraft carrier or land itself. It can be refueled and rearmed in minutes.
In case you need to penetrate an enemy airspace that is protected by SAMs, having an aircraft that needs to be refueled every Y minutes doesn't seem to be a great idea, since the Harrier does not have any good ARM, you need to get really close to the enemies to be able to shoot a missile. Being the Maverick or Sidearm. They aren't always effective, especially if the target can intercept your missiles.
As said above, range is very important. Being able to shoot stand-off weapons is better, safer and easier. Harrier doesn't have any of that, same goes for the A-10C, simply because they don't have any medium-to-long range ARMs.
F/A-18C is by far the best SEAD aircraft that we are officially getting as a module, until a F-16 arrives. (the Viper is even better because it has its own pod).
The HTS pod not only can be used to shoot ARM missiles, but it can be used to employ JDAMs, JSOWs and others.
Harrier is good for many other things, such as CAS, strike missions etc. Not for SEAD though. (Obviously I am talking about our Harrier).