If I am self-hosting, so there is no “cross-site” scripting going on, and I have a web page with an embedded PlayCanvas application (using iframe or otherwise), is it possible to call functions that are nested inside the PlayCanvas javascript files from the site’s html and javascript code?
Generally, you cannot call javascript functions from between iframes. You can use the window.postMessage browser API to send data between frames.
If you are including a PlayCanvas application in a page without using an iframe you can access the application as with normal javascript. The starting point will be getting the Application object by using var app = pc.Application.getApplication();