Brian - saw your email. What you say would be useful. Both products provide a way of import / export data so there is a way to this. Look at the Zoho api - https://www.zoho.com/projects/help/rest-api/projects-api.html
But if I was coding this I would setup a platform that allows plugins to be used so that the one integration method worked on a multiple project platforms such as basecamp etc. Possibly this the middleware could be Zapier, but in the past I have written code for something like a CORBA broker. So it becomes possible to integrate Omniplan into any platform through using a middleware platform, an XML data exchange and having field mappings acheived through connectors. Even something like the IBM Websphere MQ would allow this model to be built and Omniplan could sell the integration through subscription.
I could come up with some code but I do not have the time to take on the headache of creating this as a script or plugin to omniplan.
Omniplan is a great product, superior to a lot of other products, but is lacking in not having a cloud solution that allows sync to other products on the market. The pub/sub model is useful but only to a point.
One of the issues I have with Zoho is that there is no offline client and the assumption is made that you are always connected which I am not. Many other cloud solutions assume the same. I would prefer a fat client solution.