Whenever you're worried about using a model in a tournament, the one (and really, only) thing you should be doing is asking the TO (Tournament Organizer) if you can use the model. That said, here are some guidelines that any reasonable TO is going to use, and how they apply to your Plague Hulk.
1. Will it confuse people?
If someone is confused by what the other guy is playing, it makes it harder to play, which is a bad thing. Sometimes something is confusing because it doesn't look like anything in the game (i.e. this model jeep is a Truck). This is bad, but can sometimes be acceptable if it is fairly easy to tell what it is supposed to be. Sometimes something is confusing because it looks like something else in the game (i.e. these Chimeras with Orky things on them are Trucks). This is worse, but again, if it is fairly easy to tell what they're supposed to be, it can sometimes be acceptable. Lastly, sometimes things are confusing because they look like other things in the same army (i.e. this meltagun is a meltagun, but this other meltagun is a plasmagun). This is basically never acceptable.
The Plague Hulk looks like a Soul Grinder, so you shouldn't have any problems here.
2. Does it effect the rules?
This generally means 'is it the right size', but could also mean 'are the guns in the right place' for vehicles or 'is it modeled in a strange pose' for infantry and similar sorts of things. In 40k, it matters how big vehicles are, and where they have their guns, and how big infantry are, and whether they're lying down or not, etcetera. If your models are different than standard models, the game will play differently, and this can be a problem, especially if you are getting an advantage from the difference. The best way to deal with this is to have one 'standard' model on hand. For example, suppose that all your Rhino's are converted IG Chimeras. Someone is shooting at one and you say you get cover and he says 'well, if you had a real Rhino there, you wouldn't get cover'. You can get your Rhino out, place it where it should be, and go from there. If you're prepped like this, and clear it with the TO, you shouldn't have a problem.
Kinda hard to tell from the picture on the FW site, but I'm thinking the Plague Hulk is the same size as a Soul Grinder, so no problem there.
3. Is it cool?
The cooler something is, the more likely a TO will be to accept it. As much as WAAC jerks like myself might hate it, 40k has a huge 'hobby' component to it, and people are generally willing to play against things that are awesome looking even if they aren't perfectly WYSIWYG.
That thing looks pretty cool. Particularly if you have it in a Nurgle-themed army.
4. Is it GW?
This doesn't matter so much, particularly now that GW has pretty much gotten rid of official support for tournaments, but some TOs will be unhappy if you use non-GW models. This is particularly problematic if you're using entirely non-GW models for an entire army.
FW is more or less GW, so no problems there.
I'd check with Chase (or Sam), but I'll go on the record as stating they're stupid jerks if they don't let you use it.
