Can I submit a listing to the MLS that says that the seller is tentatively planning to review offers on a certain date, but reserves the right to accept an offer prior to that date?

No. It is an “either this OR that” situation. Either offers are being presented to the seller objectively and as quickly as possible, or offers are being held (and not discussed with the seller) until the date posted on the MLS. If there is any possibility whatsoever that the seller would like the flexibility to accept an offer quickly, then please do not post a date on the MLS.