Can't wait to look into this. Planscope (
https://planscope.io) has been syncing time with Harvest for a while, first with the Harvest Javascript SDK and now everything occurs server-to-server via their API. This latter approach has actually worked out well - because we integrate with other invoicing tools (Freshbooks, FreeAgent, Freckle, Toggl) having a consistent experience for the end user has been a must.
One thing that isn't immediately clear is whether the Harvest Platform will report back with an event when a timer is stopped from within Harvest. This routinely trips up customers, as Harvest doesn't have webhooks currently.
Regardless, CONGRATS on getting this out the door. This is going to be a huge value add for your customers.