Assuming your standard definition of market value contains exposure time, what reasonable exposure time is allowed in an open market when there is only one potential buyer?.....
The way I am understanding the OP is that there is a parcel that is not buildable because of zoning and it is in between two other parcels, and possibly having a parcel behind it.
I am also assuming that he is misusing the term "land locked" incorrectly. From the way I interpret it he has:
HOUSE............vacant parcel..............HOUSE.
So what he has is at least two potential buyers and maybe a third buyer behind the vacant parcel with the vacant parcel having road access but being unbuildable.
If I were appraising this property (and have done numerous ones similar) I would not be using existing home sales and try to extract land value differences for my analysis but rather sales of unbuildable lots. They are out there, they exist. I would also, if they are available, find vacant land sales.
But let us say there are no vacant land sales. I still would be willing to bet that there are many sales of unbuildable lots in most markets. These sales will not be in the MLS and one has to go to the assessor or auditor to find these sales which are often referred to as property line transfers in parts of the country.
If comparable properties are not found, ie the unbuildable lot sales are in superior or inferior locations one can extract a percentage impact on the unbuildable land.
Like I said, I have done multiple appraisals on these types of properties and do not do them cheap. The last one was a 40' x 100' parcel where the zoning had changed to 50' minimum buildable. I quoted a fee and the guy said I was too expensive. A week later or so he called back and said I was too expensive but no one else would do it.
A good relationship with the assessors is the key to the assignment, the MLS is no help for this type of assignment in my area (other areas might be different).