The placeholder text in the comments box currently reads;
"Let us know if you have any special requests and we will do our best to assist. Please note that special requests cannot be guaranteed."
We do not mention dietary requests specifically as not all restaurants cater for these.
If the customers are not adding these to the comments box then we could make a prominent message display in confirmation and reminder emails but can't unfortunately customise this text for you.
If you do want to go with the addition of the text to email notifications then contact our support team with the specific text and they would be happy to include.