You can put this on your reservation but there is no guarentee that your room will be ready. I was actually told by CRO that it is a good thing to have on there because they will at least attempt to have a room ready for you. I didnt for our Easter trip and 6.5 hours after we checked in we finally got into our room and that was after speaking with the resort manager and having them move what room we were in. It was well past the 3:00 check in time when I went to her.
If your room isnt ready the Polynesian can hold your luggage for you so that you can go to the parks or swimming then deliver it to your room later.
I have been there in September the past 2 years and have had no trouble getting in the room early (actually... by 11:00 am). I did not put it on the request - I guess it depends on what time of year you go.
I was lucky this past June, we go to our 1st hotel, AKL at noon time and got our room right away, then after one night we moved to Poly and our room was ready around 11:30 but it doesn't end there, after that we went to BWI and our room was ready when we got there at noon time again. I just lucked out or is that in, maybe it won't happen next yr. but this year it was great.
Disney changed the room allocation procedure about a year ago. For the most part rooms no longer pre-assigned. The desk clerk is supposed to give you an available room that best suits your requests. Much more likely to get a room when you check-in.