Does VA have a provision for write in candidates?
NO, not in the primary.
Write-in votes are allowed (but not required to be allowed) in a primary election. It is up to the party to decide whether to allow write-in votes.
The the 2008 presidential primary election the Virgina Republican Party counted zero (0) write-in votes.
It is up to the Virgina Republican Party to decide whether or not to count write-in votes for 2012.
Given that party the VA GOP is dominated by Romney supporters, it is very doubtful.