Hi Susan
Yes I understand that you could make any arrangement with a dealer to reserve you a car for a later date and of course the arrangement may involve a non returnable deposit.
However, as you say, Motability will only get involved contracturally with you when your permitted order date comes around. Obviously you will need to pay the AP that is applicable at that time and if you have an agreement with a dealer too far in advance you take the risk that you may not be able to lease the car because it is off the Motability scheme.
Apart from finding a dealer willing to agree to this type of order there must be risk to this otherwise everybody on the scheme will be ordering in advance to avoid the build lead in.