You can get your luggage transferred for you by the resorts. You just have to call them and they will pick it up from your room. You can go to the parks for the day and return and check in later at the BC. You then can call and get your luggage delivered to your room there. This way you miss no actual park time.
If you weren't planning on doing a park that day, you can always take the bus to MGM and walk to the BC from there (about a 10-15 minute walk) or take the boat, which will take about as long and just have your luggage shipped. You can check in and if you carry your bathing suits, enjoy stormalong bay all day long