My experience (datapoint 1, working on ecocar in college a decade ago) was that they are exceedingly defensive revealing info about the CAN message format. It would likely require reverse engineering the protocol in order to make any apps that actually interface with the car somehow. Making a fully standalone app would be possible, but limited in usefulness.

That depends on what you want to do. OBDII diagnostics is legally mandated by law and so not hard to do across all cars. It is minimal information, but enough for most things. If you want anything else it different for each car (not just each manufacture, sometimes sometimes the same car model will have more than one variant).

The information you need isn't impossible to get: the manufactures make this available to third party scan tools, but they assume anyone wanting this information considers paying $100,000 a nominal cost and won't question it. In a previous job (I left in 2010) I had access to this information, but we still spent a lot of time reverse engineering things because the documents were too often wrong.

See:

* DBC file is a proprietary format that describes the data over a CAN bus.: http://socialledge.com/sjsu/index.php/DBC_Format

* OpenDBC: democratize access to car decoder rings: https://github.com/commaai/opendbc

OBDII is only required for engine diagnostics, no? I've heard that Tesla cars don't have them, for example.

It depends what country you're in. My country, the Tesla does have an ODB II port, because it's a requirement for cars sold here. I don't believe that's the case in the US, although those have a CAN port.

It is only required for emissions diagnostics. Which is a small subset of total engine diagnostics, though the most common things to go wrong on an engine affect emissions so for practical purposes it is engine.