Awesome! I already tinkered with a similar idea, called "Sync" module. I think you've done the ground-work for this with Deployment now. :)

Currently, I'm developing Migrator module, which aims to convert any existing (non-Drupal) site to Drupal. To accomplish this, Migrator creates an object id mapping table to ensure that external ids can be altered to newly created Drupal ids (f.e. for users, user roles, nodes, taxonomy terms, aso) in front of creating the imported objects in Drupal. IMHO, a similar id mapping mechanism could work out for Deployment, too, since you already need to process the pushed data on the remote site.