JLT, we also live in the Herne Bay area, I think this is a highly unlikely scenario of not getting into a grammar school, especially when Chatham House and Dane Court struggle to fill places and take a high percentage on appeal.
Two years ago Herne Bay, Whitstable and even parts of Canterbury lost out on GS places but as far as I know this was the only year it happened(?) so please bear in mind that it can and does happen.
It's a bit of a Catch 22 - if you put all Grammar schools on your SCAF and nowhere can offer you a place, you will end up at your nearest school regardless of whether or not it is a grammar. If you put Three GS choices and a HS choice, then you could still miss out on a possible GS place which you could have had as your fourth choice.
IF you were offered the HS choice and it went to appeal for a GS place, then the panel will know as they will have a copy of the offer letter you would have been sent. This is what happened to me two years ago and the paperwork sent to me that the appeal panel had copies of included the offer letter and the letter assessing DS as suitable for GS a well as copies of the paperwork I had submitted to them.
Which schools are you thinking of JLT?