This is a quick one because I know there will be many people like me who are asking the same question but may not have the answer to the above.
Also what if your ROM date is set at more than the stipulated 3 months from date of collection, in which case you can't even register yourself at the ROM?
The answer is yes, most likely you can still collect your keys, provided you inform the HDB in writing beforehand, simply by emailing the customer relations manager in charge of your appointment (stated in the letter) and getting their approval prior to the day of key-collection. At the same time, you should be able to show them real hard evidence that you are indeed preparing for your wedding, i.e receipt of rental of your wedding venue, correspondance with the Justice of Peace, etc.
The underlying point is you must have THE intention to be married if you are not already. The last thing HDB wants is to catch you conning them of a flat, lol.
Upon written approval from the HDB side, then you get your keys on the appointed date. But do remember to later submit a copy of the marriage certificate by the date instructed on the deferment notice!