The amount of preservative that would leach from your reclaimed wood is minimal.
Since the wood has been previously exposed to the elements, most of the leaching would have occurred.
You would actually be adding chemicals to the wood by using an additional coating.
For more information:
Read "Are there nontoxic stains that would be safe for wood planters in a veggie garden?" a Q&A answered by Debbie Sek.
Also, check Polly Osborne's answer to the Q&A "Can you suggest an alternative to pressure-treated wood for garden borders and raised planters?"