A bill was recently introduced in Congress by Rep. McDermott (D-Washington) to allow a credit for 30% of qualified expenses (i.e., legal fees incurred establishing the advance directive) up to a cap of $500. The bill is freshly introduced, so there is not telling whether it will make it through all the committees and votes to become law. It sure will be interesting to follow.
The text of the bill HR 2705 can be found here