I believe if you really want it to hold up, you should use the Bellcore document signing service. Has anyone heard of a company that would provide this on a non-research basis?

The Bellcore service is properly a timestamping service and not a signature service. Their timestamp is constructed out of hash functions, not digital signatures. The algorithm is patented. Contact Bellcore for licensing. I'm not sure they are going to license; they may decide that they want all the timestamping revenue themselves. Eric