I do a few of these every year.
The good news is that 300 ppl is not too big at all. Are you responsible for covering a "Step & Repeat" on the red carpet, and the general candids? If so, I would strongly suggest you have at least two competent shooters. This way you can have one working the red carpet and the Step & Repeat, and then one capturing general candids as everyone mills about. If your doing onsite portraits, I would pick your location, and then have the event coordinator identify it for the guests, so they know where to go for their portraits. Personally, I would try to make them get rid of the portrait set up and instead, stick with the S&R, and general event candids. If they are not doing the S&R, you may be able to get away with setting up a small backdrop at the end of the red carpet to do a quick portrait as they come in.
Be cautious about
"them setting aside time" as generally this is far too little time to do much of anything, because photography is not on their mind, and they have no concept of how long anything takes. Providing you with far too little time to do a good job, and them being unhappy with the results can go hand in hand, so make sure you spell out the expectations you need far in advance.
As previously mentioned, the time does go by quickly so make sure they will have someone from
PR there alongside you. This ensures you nail those ever important shots of people you will not be able to identify. Even very famous people look totally different when your shooting a sea of faces in rapid succession.
Good Luck,
W