The PNaCl ABI is defined here: https://developers.google.com/native-client/dev/reference/pn... - and it's definitely a work in progress we'll be trying to improve. As for Pepper, it's also all documented here and below - https://developers.google.com/native-client/dev/pepperc/

Granted, this is documentation, not a standards-body spec.

Yes, certainly has had effort put into proper documentation. Do you know if there is also work being done to spec it formally?

We're definitely considering it.