I saw your post on techcrunch about this and I thought I would let you know that you can do what you want right now at Middlepost.

“(a) provide each party to the deal with login information and a way to authenticate individual identities.”

You can invite everyone that needs to be involved in the document (Observers, Approvers, and Signers) into the document space on Middlepost. From there, people can read, review, upload revisions, comment, and when everyone is happy, sign it.

“(b) somehow coordinate my digital signatures with the other side’s, or some combination thereof.”

Both sides (or even if there are several sides involved) access and sign the same document so it’s inherently coordinated. Also, after a document is signed and completed, we lock it and store it for a minimum 7 years so nobody can change the document and any side can come back and find the document at a later date.

Please feel free to contact me if you would like to discuss.